Optical disk device and optical disk

ABSTRACT

The invention provides an optical disk apparatus and an optical disk that are applicable to a multilayer optical disk, and enable to realize tilt detection with a high precision. The optical disk has a transparent planar disk base member, a recording layer formed on the disk base member, and a reflecting layer in a certain positional relation to the recording layer. The optical disk apparatus includes a laser pointer  61  for irradiating a laser beam onto the recording layer of the optical disk by way of the disk base member to form a focusing spot on the recording layer, a photo-sensor array  6 G for receiving a reflected beam from the reflecting layer, and an aberration mode detecting circuit  6 H for detecting tilt of the optical disk by using an output from the photo-sensor array 6G.

TECHNICAL FIELD

The present invention relates to an optical information recording mediumsuch as an optical disk, an optical disk apparatus for recording orreproducing optical information on or from an optical informationrecording medium, and an optical disk.

BACKGROUND ART

It is effective to increase the numerical aperture of an objective lensand to shorten a laser wavelength for maximizing the capacity of anoptical disk. Also, in recent years, development of a multilayerrecordable optical disk has been progressed. In multilayer recording, itis important to keep a laser beam from attenuating due to absorption ordispersion of the laser beam on a recording layer formed between a diskbase member and a target recording layer in irradiating the laser beamonto the target recording layer via the disk base member. In view ofthis, there is proposed a technique of reducing unnecessary absorptionand dispersion of a laser beam on a site other than a focusing spot byutilizing a nonlinear optical effect such as two-photon absorption.

Aberration by tilt is one of the drawbacks involved in maximizing thecapacity of an optical disk. Tilt throughout the specification and theclaims means tilt of an optical axis of a laser beam with respect to anormal line to a surface of a substrate of an optical disk. If thenumerical aperture of an objective lens is increased, and the laserwavelength is shortened, an influence of aberration due to tilt of anoptical disk is increased. In some cases, a substantial thickness of adisk base member is increased if recording is attempted onto a layerproximate to the bottom of the optical disk in multilayer recording,with the result that an influence of aberration by tilt is significantlylarge. Such an aberration fails to obtain a clear focusing spot, andlowers reliability in recording/reproducing. Accordingly, in recordinginformation of a large capacity onto an optical disk, it is essentiallyimportant to accurately detect tilt of the optical disk.

Aberration by tilt includes odd symmetrical aberration such as comaaberration and astigmatism. The following fact is known in an opticalsystem of forming a focusing spot on a flat disk substrate such as anoptical disk. If aberration has occurred in an incoming optical path,such an aberration may be cancelled in an outgoing optical path.Therefore, it is impossible to detect tilt of the optical disk simply bymeasuring aberration of a reflected beam from the focusing spot. This isone of the problems to be solved in tilt detection.

A first example of the conventional tilt detection is disclosed, forinstance, in Japanese Unexamined Patent Publication No. 11-232677(called as “D1”). In this example, after a signal from a detector isdivided into two components in tangential directions of an optical disk,the signal components are differentially amplified to generate atangential push-pull signal. Front and rear edge portions of a mark on arecording layer are detected with use of the tangential push-pullsignal. Tilt of the optical disk in the tangential directions isdetected based on a symmetrical property of a crest value of thetangential push-pull signal at the front and rear edge portions of themark. In D1, similarly to the tangential push-pull signal, a radialpush-pull signal is generated in radial directions of the optical disk,and tilt of the optical disk in radial directions is detected based on asymmetrical property of the radial push-pull signal at the front andrear edge portions of the mark.

A second example of the conventional tilt detection is disclosed, forinstance, in Japanese Unexamined Patent Publication No. 2003-77158(called as “D2”). D2 discloses a simpler method of detecting tilt of anoptical disk in tangential directions. According to this method, areproduction signal is inputted to a differentiating circuit, an outputfrom the differentiating circuit is compared with a predetermined levelby a comparator circuit, and a pulse width of the output from thecomparator circuit is measured to perform tilt detection. Similarly tothe first example, in the second example, front and rear edge portionsof a mark on a recording layer are detected, and tilt of an optical diskis detected based on a symmetrical property regarding the front and rearedge portions.

A third example of the conventional tilt detection is disclosed, forinstance, in Japanese Unexamined Patent Publication No. 2003-16680(called as “D3”). In D3, tilt is detected in the following manner. Adefocused state is created by adding an offset component to a focuscontrol signal through application of an offset voltage to a defocusdetection signal when an objective lens is focused. A tracking errorsignal detected in the defocused state is extracted as a tilt signal ina radial direction of an optical disk.

The aforementioned conventional optical disks and optical diskapparatuses have suffered from the following drawbacks.

In the first and the second conventional examples, a groove or a pit isrequired to be formed in tilt detection. If the techniques disclosed inD1 and D2 are applied to multilayer recording, diffraction or dispersionmay occur due to the existence of the groove or the pit formed in eachof the recording layers, which may unduly reduce the light amount to bereceived on the optical disk.

Further, in these conventional tilt detections, when an optical disk istilted, a side robe is generated in skirt portions in a tilted directionof a beam spot and in a direction opposite to the tilted direction. Theconventional techniques utilize a phenomenon that a crest value of adifferentiated waveform of a push-pull signal or a reproduction signalis decreased due to generation of the side robes.

When the tilt angle is small, the magnification of the side robe issmall. Accordingly, sufficient tilt detection sensitivity in atangential direction of an optical disk cannot be expected. Also, sincea laser beam does not propagate in a radial direction of an opticaldisk, tilt detection sensitivity in the radial direction is smaller thanthat in the tangential direction. Therefore, the S/N ratio of adetection output concerning the tilt detection is significantly small,which resultantly leads to a low precision in tilt detection.

The third conventional example has a drawback that recording/reproducingis impossible while the optical disk is in a defocused state. It takesquite a long time to create a defocused state by moving an objectivelens for tilt detection, move the objective lens again to focus onto atarget recording layer, and to read an address from the recording layerto confirm that the objective lens has returned to the target recordinglayer. This arrangement lacks real-time responsiveness in tiltdetection.

DISCLOSURE OF THE INVENTION

In view of the above problems residing in the prior art, it is an objectof the invention to provide an optical disk apparatus that is compatiblewith a multilayer optical disk, and is capable of performing tiltdetection of a high precision, and an optical disk.

To solve the above problems, an optical disk apparatus according to anaspect of the invention comprises: a light source which irradiates alaser beam onto a recording layer of an optical disk by way of a diskbase member to form a focusing spot on the recording layer, the opticaldisk having the transparent planar disk base member, the recording layerformed on the disk base member, and a reflecting layer in a certainpositional relation to the recording layer; a photo detector whichreceives a reflected beam from the reflecting layer; and a tiltdetecting means which detects tilt of the optical disk by using anoutput from the photo detector.

In the above arrangement, the reflecting layer parallel to the recordinglayer on which the laser beam is focused is formed, and tilt of therecording layer is indirectly detected by using the reflected beam fromthe reflecting layer. Since the reflected beam from the reflecting layeris defocused, tilt aberration and coma aberration are not cancelled.Forming the reflecting layer on the optical path enables to keepdetection sensitivity in tilt detection from lowering due to offset ofaberration of the laser beam between the incoming beam and the outgoingbeam, thereby enabling to perform tilt detection of a high precision.Also, this arrangement enables to detect tilt of the optical diskwithout forming a groove or a pit in the recording layer of the opticaldisk, which is advantageous in effectively suppressing lowering of thelight amount to be received on the optical disk due to diffraction ordispersion of the laser beam on a recording layer other than the targetrecording layer in multilayer recording where recording is executed withrespect to multiple recording layers.

These and other objects, features, aspects, and advantages of thepresent invention will become more apparent upon reading of thefollowing detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C are illustrations for explaining a principle of tiltdetection in a first embodiment of the invention.

FIG. 2 is a cross-sectional view showing an optical disk in the firstembodiment.

FIG. 3 is an illustration showing an arrangement of an optical diskapparatus in the first embodiment.

FIGS. 4A through 4C are illustrations for explaining a principle of tiltdetection in a second embodiment of the invention.

FIG. 5 is an illustration for explaining an arrangement of a recordinglayer of an optical disk in the second embodiment.

FIGS. 6A and 6B are illustrations for explaining an arrangement of therecording layer of the optical disk in the second embodiment.

FIG. 7 is a cross-sectional view of the optical disk in the secondembodiment.

FIG. 8 is an illustration showing an arrangement of an optical diskapparatus in the second embodiment.

FIG. 9 is a cross-sectional view of an optical disk in a thirdembodiment of the invention.

FIG. 10 is an illustration showing an arrangement of an optical diskapparatus in the third embodiment.

FIG. 11 is an illustration showing an arrangement of an optical diskapparatus in a fourth embodiment of the invention.

FIG. 12 is an illustration for explaining a principle of tilt detectionin a fifth embodiment of the invention.

FIG. 13 is an illustration showing an arrangement of an optical diskapparatus in the fifth embodiment.

FIGS. 14A and 14B are illustrations for explaining a principle of tiltdetection in a sixth embodiment of the invention.

FIG. 15 is an illustration showing an arrangement of an optical diskapparatus in the sixth embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, an optical disk apparatus and an optical diskembodying the present invention are described referring to the drawings.

First Embodiment

FIGS. 1A through 1C are illustrations for explaining a principle of tiltdetection in the first embodiment. Although FIGS. 1A through 1C show anexample that an optical disk has a single recording layer for sake ofeasy explanation, the same principle is applied to a case that anoptical disk has plural recording layers. FIG. 1A is an illustrationshowing an optical path of laser light (laser beam) when an optical diskis not tilted. As shown in FIG. 1A, the laser beam incident through abase member 12 of an optical disk by way of a surface 17 thereof isreflected from a recording layer 13. In other words, the incident laserbeam forms an optical path 15 passing the points A, B, C, D, and E. FIG.1B shows a state that the optical disk shown in FIG. 1A is tilted. InFIG. 1B, an optical axis 10 of the laser beam is tilted with respect toa normal line 11 to the recording layer 13 by a certain angle 16. Theangle 16 corresponds to a tilt angle. The optical path 15 shown in FIG.1B passes the points A, B′, C′, D′ and, E′. Since the length of theoptical path 15 is the same between the case that the optical disk isnot tilted and the case that the optical disk is tilted, tilt of theoptical disk cannot be detected with use of the reflected laser beam. Inthis embodiment, as shown in FIG. 1C, a reflecting layer 14 parallel tothe recording layer 13 is formed, and a laser beam is allowed topropagate through the recording layer 13 by adjusting the transmittanceof the recording layer 13 to perform tilt detection using the laser beamreflected from the reflecting layer 14. In FIG. 1C, similarly to FIG.1B, the length of the optical path 15 passing the points A, B′, C′, D′,and E′ is the same as that of the optical path 15 passing the points A,B, C, D, and E in FIG. 1A. However, the length of the optical path 15passing the points A, B′, C′, C″, D″, and E″ in FIG. 1C is obviouslydifferent from the length of the optical path 15 passing the points A,B′, C′, D′, and E′ shown in FIG. 1B. Therefore, applying the idea shownin FIG. 1C enables to detect tilt aberration and coma aberration byusing the reflected beam without cancellation of these aberrations dueto increase/decrease in optical path length between the incoming opticalpath and the outgoing optical path.

FIG. 2 is an illustration showing a cross section of a multilayeroptical disk in the first embodiment. The optical disk has an upper basemember 51, a recording layer stack 52, a reflecting layer 53, and alower base member 54. The recording layer stack corresponds to a portionwhere a number of recording layers 55 are formed one over the other viaan intermediate layer 56. The respective recording layers 55 in therecording layer stack 52 are formed parallel to the reflecting layer 53.These parallel layers are formed, for instance, by forming oneintermediate layer 56 on the reflecting layer 53 by spin coat orsputtering, and by forming a recording layer 55 and then anotherintermediate layer 56 by spin coat or sputtering. Thus, the recordinglayers 55 and the reflecting layer 53 are formed parallel to each other.

The gap between the adjoining recording layers 55, namely, the thicknessof each intermediate layer 56 is e.g. 10 μm. The thickness of theintermediate layer 56 is determined by the number of data recorded in acircle 5B, which is defined by laser beams 57 propagating in acrisscross manner through an adjoining recording layer 5A adjoining atarget recording layer 59 on which the laser beams 57 are focused. Thisis for the following reason. A change of the numerals “1” and “0”, whichare recording codes representing the data recorded in the circle 5B, isread as a crosstalk noise. As the number of data recorded in the circle5B is increased, the change is averaged. Accordingly, a predeterminedallowable crosstalk noise determines the number of data recorded in thecircle 5B, and the number of data recorded in the circle 5B determinesthe gap between the adjoining recording layers 55, namely, the thicknessof the intermediate layer 56. Preferably, the intermediate layer 56formed between the lowermost layer of the recording layer stack 52 andthe reflecting layer 53 is 10 μm or less e.g. about 3 μm, because thereis no data recorded on the reflecting layer 53.

A reflecting layer used with a recording layer in a conventional opticaldisk serves as a light reflecting layer for allowing a laser beam to beirradiated onto the recording layer a number of times, and as a heatdiffusing layer for readily diffusing heat generated on the recordinglayer. In view of this, the recording layer and the reflecting layer areformed proximate to each other, so that a laser beam reflected from thereflecting layer is efficiently irradiated onto the recording layer, orheat generated on the recording layer is efficiently diffused by thereflecting layer (see Matsushita Technical Journal Vol. 45 No. 6December 1999 pp. 672-678, Section 3.2 “Disk Design Technology”).

In the case that an optical disk is a phase change disk such as aDVD-RAM disk, the gap between the recording layer and the reflectinglayer is about 20 nm to about 200 nm (see Technical Report of IEICECPM2000-95 2000.09 pp. 21-27, Section No. 4: Computation Result).

On the other hand, the reflecting layer 53 of the optical disk in thisembodiment is formed to reflect a laser beam that has been focused onthe recording layer 55 and has been defocused on the reflecting layer53. In other words, a certain degree of defocusing is required. Unlessotherwise, tilt aberration or coma aberration in the reflected laserbeam may be cancelled in the outgoing optical path, thereby loweringdetection sensitivity in tilt detection.

In view of the above, it is necessary, in this embodiment, to set thegap between the recording layer 53 (sic) and the reflecting layer 55(sic), namely, the thickness of the intermediate layer 56 to be formedbetween the recording layer 53 (sic) and the reflecting layer 55 (sic)sufficiently larger than the wavelength of a laser beam. For instance,in the case where a laser beam of a wavelength of 660 nm is used, thegap between the recording layer 53 (sic) and the reflecting layer 55(sic) is required to be set five times as large as the wavelength of thelaser beam, namely, about 3,000 nm (3 μm) or more.

As mentioned above, the gap between the recording layer and thereflecting layer in the embodiment of the invention is about several tentimes to several hundred times as large as that in the conventionaloptical disk, and the constructions of the optical disks in theembodiment and in the conventional art are different from each otheraccordingly.

A photochromic material such as diarylethene or fulgide is used forforming the recording layers 55. A UV curing resin, ZnS—SiO₂ or a likematerial is used for forming the intermediate layers 56, for instance. Asilicon thin film or a metallic thin film made of aluminum or a likematerial is used as the reflecting layer 53, for instance.

It is possible to form the recording layers 55 of a photoisomerizingmaterial having a property that two-photon absorption occurs byirradiation of laser beam. Two-photon absorption is one of nonlinearoptical effects, wherein a molecule of a material simultaneously absorbstwo photons, thereby changing a refractive index or a like factor of thematerial. The refractive index of a focusing spot on an optical diskmade of the photoisomerizing material where a laser beam is to befocused can be changed by utilizing the two-photon absorption. In amultilayer optical disk, a target recording layer for which recording isperformed can be selected by controlling the focal point of a laser beamin a depthwise direction of the optical disk. An example of thephotoisomerizing material is diarylethene.

FIG. 3 is an illustration showing an arrangement of an optical diskapparatus in the first embodiment. An optical disk 66 is the multilayeroptical disk shown in FIG. 2. A laser pointer (light source) 61 isdriven by a laser driving circuit 60 to output a laser beam of a certainpower. The laser beam outputted from the laser pointer 61 is convertedinto parallel rays by a collimator lens 62.

Spherical aberration of the parallel rays of the laser beam is correctedby a deformable mirror 6Q. A correction amount of the sphericalaberration to be carried out by the deformable mirror 6Q is determinedin such a manner as to minimize a spherical aberration amount in a laserbeam reflected from the recording layer 59 on which the laser beams 57are focused through the recording layer stack 52 in FIG. 2. Thespherical aberration amount in the reflected laser beam from therecording layer 59 is detected by an even symmetrical aberration sensor6S. The even symmetrical aberration sensor 6S is a sensor for outputtingan aberration amount of modal aberration of an even order in a Zernikemode such as defocus aberration and spherical aberration, for instance.The spherical aberration output from the even symmetrical aberrationsensor 6S is temporarily inputted to a servo controller 6U. The servocontroller 6U drives the deformable mirror 6Q by way of a deformablemirror driving circuit 6R based on the spherical aberration amountdetected by the even symmetrical aberration sensor 6S.

The laser beam reflected from the deformable mirror 6Q is incident ontoan objective lens 64 after passing through a deflecting beam splitter 63and a quarter (¼) wavelength plate 6T. The servo controller 6U controlsan objective lens actuator 65 based on the defocus aberration amountoutputted from the even symmetrical aberration sensor 6S to move theobjective lens 64 in such a manner that the laser beam is focused onto atarget recording layer in a recording layer stack 67.

A part of the laser beam that has reached the target recording layer inthe recording layer stack 67 reaches a reflecting layer 68 through therecording layer stack 67. The other part of the laser beam that hasreached the target recording layer in the recording layer stack 67 isreflected from the target recording layer in the recording layer stack67. The reflecting layer 68 is made parallel to the recording layers inthe recording layer stack 67. The laser beam that has reached thereflecting layer 68 is reflected thereon and is returned to theobjective lens 64. The laser beam that has returned to the objectivelens 64 is passed through the objective lens 64 and the quarterwavelength plate 6T, is reflected from the deflecting beam splitter 63in a direction different from the direction of the incoming beam, and isincident onto a half mirror 6V. The laser beam incident onto the halfmirror 6V is split into two laser beams by the half mirror 6V. One ofthe laser beams split on the half mirror 6V, namely, the laser beampropagating through the half mirror 6V is incident onto the interior ofa tilt sensor 6P indicated by the dotted line block in FIG. 3. The otherof the laser beams split on the half mirror 6V, namely, the laser beamreflected from the half mirror 6V is incident onto the interior of theeven symmetrical aberration sensor 6S indicated by the dotted line blockin FIG. 3.

The tilt sensor 6P is an improved sensor of a conventional well-knownmodal wavefront sensor. A modal wavefront sensor is a wavefront sensorfor outputting a wavefront in terms of coefficients in an orthogonalaberration mode such as a Zernike mode. The modal wavefront sensor has afeature that an aberration amount of a predetermined aberration mode isdetectable irrespective of an aberration amount of the other aberrationmode. The modal wavefront sensor can detect an aberration amount of comaaberration, which is one of the aberration modes, independently of theother aberration modes e.g. spherical aberration. In this arrangement,tilt of an optical disk can be detected by detecting tilt aberration orcoma aberration with use of the modal wavefront sensor.

A modal wavefront sensor based on which the embodiment of the inventionhas been made is recited in New modal wave-front sensor: a theoreticalanalysis, J. Opt. Soc. Am. A/Vol. 17, No. 8 (sic), pp. 1098-1107/June2000 by Mark A. A., Tony Wilson, et al.

The modal wavefront sensor recited in the above document and the tiltsensor 6P in this embodiment are different in the arrangement that thelatter is additionally provided with a mechanism for canceling defocusaberration and spherical aberration of a laser beam incident onto thetilt sensor 6P. The mechanism for canceling defocus aberration isrealized by making a condenser lens 6D movable. The mechanism forcanceling spherical aberration is realized by providing a deformablemirror 6A.

The laser beam incident on the tilt sensor 6P is incident onto thedeformable mirror 6A. The deformable mirror 6A has its mirrorconfiguration changed depending on an inputted spherical aberrationcontrol signal 6M. As mentioned above, spherical aberration is canceledby the deformable mirror 6A.

The laser beam reflected from the deformable mirror 6A is incident ontoa hologram 6C. Eight kinds of bias aberrations, namely, two kinds ofbias X coma aberrations which are different in sign and identical insize, two kinds of bias Y coma aberrations which are different in signand identical in size, two kinds of bias defocus aberrations which aredifferent in sign and identical in size, and two kinds of bias sphericalaberrations which are different in sign and identical in size are addedto the incident laser beam in the hologram 6C. The aberration amounts ofthe respective bias aberrations are determined by the detectedaberration amounts, and preferably set to about half of the respectivedetected aberration amounts.

The laser beam added with the respective bias aberrations in thehologram 6C is incident onto the condenser lens 6D. The condenser lens6D is supported by a condenser lens actuator 6E. The condenser lensactuator 6E moves the focus position of the condenser lens 6D dependingon a defocus aberration control signal 6L. As mentioned above, defocusaberration is canceled by moving the condenser lens 6D.

The laser beam incident onto the condenser lens 6D is focused on apinhole plate 6F. Eight pinholes are formed in the pinhole plate 6F incorrespondence to the number of the added bias aberrations. The radiusof each of the pinholes is for instance 1/1.22 times as large as theradius of an airy disk.

The laser beams that have passed through the pinholes in the pinholeplate 6F are incident onto photo-sensors of a photo-sensor array 6Garrayed in correspondence to the pinholes, respectively. The laser beamsincident onto the respective photo-sensors are converted into electricalsignals, which are outputted to an aberration mode detecting circuit 6H.A signal from each of the photo-sensors is differentially amplified inthe aberration mode detecting circuit 6H with respect to each of theaberration modes. Specifically, the aberration mode detecting circuit 6Houtputs X-,Y-tilt detection signals 6N (X-,Y-coma aberration detectionsignals), a defocus aberration detection signal 6J, and a sphericalaberration detection signal 6K. The X-,Y-tilt detection signals 6N(X-,Y-coma aberration detection signals) constitute an output from thetilt sensor 6P.

The defocus aberration detection signal 6J, the spherical aberrationdetection signal 6K, and the X-,Y-tilt detection signals 6N areoutputted to a defocus aberration/spherical aberration cancel controller6I. The defocus aberration/spherical aberration cancel controller 6Igenerates the defocus aberration control signal 6L and the sphericalaberration control signal 6M based on the defocus aberration detectionsignal 6J and on the spherical aberration detection signal 6K in such amanner as to cancel defocus aberration of the laser beam incident ontothe condenser lens 6D and to cancel spherical aberration of the laserbeam incident onto the deformable mirror 6A, and outputs the defocusaberration control signal 6L and the spherical aberration control signal6M to the defocus aberration/spherical aberration cancel controller 6I.Simultaneously, the defocus aberration detection signal 6J, thespherical aberration detection signal 6K, and the X-,Y-tilt detectionsignals 6N are outputted to the servo controller 6U. The servocontroller 6U and the defocus aberration/spherical aberration cancelcontroller 6I are communicated to each other by an interactivecommunication line.

The tilt sensor 6P having the above arrangement has the followingfeatures, as compared with the modal wavefront sensor recited in theabove document.

Coma aberration in the laser beam reflected from the reflecting layer 68is not canceled, and remains therein. Theoretically, tilt of the opticaldisk is detectable by using the coma aberration. However, defocusaberration and spherical aberration, which are significantly largeaberrations, also remain in the reflected laser beam, as well as thecoma aberration. Accordingly, if the modal wavefront sensor recited inthe document is used, a clear beam spot is not formed even with use ofthe condenser lens 6D, with the result that a detection output may beunduly reduced.

On the other hand, in the case where the tilt sensor 6P is used, afterdefocus aberration and spherical aberration of the laser beam incidentonto the tilt sensor 6P are canceled, the laser beam is incident ontothe condenser lens 6D. This arrangement enables to obtain a clear beamspot with a sufficiently large Strehl ratio and to obtain X-,Y-tiltdetection signals 6N with a high S/N ratio, namely, with a largedetection output.

Now, an arrangement of the even symmetrical aberration sensor 6S isdescribed.

The laser beam reflected from the target recording layer in therecording layer stack 67 is returned to the objective lens 64. The laserbeam that has returned from the target recording layer to the objectivelens 64 is passed through the objective lens 64 and the quarterwavelength plate 6T, is reflected from the deflecting beam splitter 63in a direction different from the direction of the incoming beam, and isincident onto the half mirror 6V. The laser beam incident onto the halfmirror 6V is split into two laser beams by the half mirror 6V. One ofthe laser beams is incident onto the interior of the tilt sensor 6Pindicated by the dotted line block in FIG. 3, and the other of the laserbeams is incident onto the interior of the even symmetrical aberrationsensor 6S indicated by the dotted line block in FIG. 3.

The even symmetrical aberration sensor 6S detects defocus aberration andspherical aberration of the laser beam that has been focused on thetarget recording layer in the recording layer stack 67 and reflectedthereon. Unlike the laser beam detected by the tilt sensor 6P, the laserbeam detected by the even symmetrical aberration sensor 6S does notinclude a large defocus aberration or a large spherical aberration.Therefore, there is no need of canceling defocus aberration or sphericalaberration of the laser beam detected by the even symmetrical aberrationsensor 6S. In this sense, the even symmetrical aberration sensor 6S isequivalent to the modal wavefront sensor recited in the document.

The laser beam incident onto the even symmetrical aberration sensor 6Sis incident onto a hologram 6W. Four kinds of bias aberrations, namely,two kinds of bias defocus aberrations which are different in sign andidentical in size, and two kinds of bias spherical aberrations which aredifferent in sign and identical in size, are added to the incident laserbeam in the hologram 6W. Aberration amounts of the respective biasaberrations are determined by the detected aberration amounts, andpreferably set to about half of the respective detected aberrationamounts.

The laser beam added with the respective bias aberrations in thehologram 6W is incident onto a condenser lens 6X. The position of thecondenser lens 6X is adjusted in such a manner that the laser beam fromthe focusing spot of the objective lens 64 is focused.

The laser beam incident onto the condenser lens 6X is focused on apinhole plate 6Y. Four pinholes are formed in the pinhole plate 6Y incorrespondence to the number of the added bias aberrations. The radiusof each of the pinholes is for instance about 1/1.22 times as large asthe radius of an airy disk.

The laser beams that have passed through the pinholes in the pinholeplate 6Y are incident onto photo-sensors of a photo-sensor array 6Zarrayed in correspondence to the pinholes, respectively. Thephoto-sensors convert the incident laser beams into electrical signals,which are outputted to an aberration mode detecting circuit 610. Asignal from each of the photo-sensors is differentially amplified in theaberration mode detecting circuit 610 with respect to each of theaberration modes. The aberration mode detecting circuit 610 outputs adefocus aberration signal and a spherical aberration signal to the servocontroller 6U.

A reproduction signal for reproducing recorded data is obtained byadding aberration signals of the same kind with different signs withinthe signals outputted from the photo-sensors in the tilt sensor 6P andin the even symmetrical aberration sensor 6S. For instance, areproduction signal is obtained by adding a signal from thephoto-detector which has undergone bias addition of a plus sign withrespect to defocus aberration in the even symmetrical aberration sensor6S, and a signal from the photo-detector which has undergone biasaddition of a minus sign with respect to defocus aberration. Also, areproduction signal with a high S/N ratio is obtained by adding signalsfrom plural sets of photo-detectors in place of adding signals from oneset of photo-detectors.

Now, an operation of the optical disk apparatus in this embodiment isdescribed. In an initial state of the optical disk apparatus such asturning on of the power of the apparatus, the servo controller 6Ucontrols the objective lens actuator 65 to move the objective lens 64 insuch a manner that a laser beam is focused on a target recording layerin the recording layer stack 67. An example of a control procedure fromthe initial state of the optical disk apparatus until a tilt detectionsignal is outputted is described as follows.

(1) The objective lens actuator 65 temporarily moves the objective lens64 to such a position that a laser beam is substantially focused on thesurface of the optical disk 66.

(2) The defocus aberration/spherical aberration cancel controller 6Idrives the condenser lens actuator 6E to adjust the position of thecondenser lens 6D of the tilt sensor 6P in such a manner that an“S-shaped curve” is detected based on the defocus aberration detectionsignal 6J with use of the laser beam reflected from the surface of theoptical disk 66. At the same time, the servo controller 6U controls theobjective lens 64 in such a manner that an “S-shaped curve” is detectedbased on the defocus aberration output from the even symmetricalaberration sensor 6S, and that the laser beam is focused onto thesurface of the optical disk 66. At this time, spherical aberration iscorrected with respect to the surface of the optical disk 66 in such amanner that correction amounts of spherical aberration are identical toeach other between the deformable mirror 6A and the deformable mirror6Q.

(3) When the “S-shaped curve” is detected based on the defocusaberration output from the even symmetrical aberration sensor and thedefocus aberration detection signal 6J using the laser beam reflectedfrom the surface of the optical disk 66, the servo controller 6U movesthe focusing spot of the laser beam in a downward direction from thesurface of the optical disk 66 by moving the objective lens 64. Theservo controller 6U controls the deformable mirror 6A, the deformablemirror 6Q, and the condenser lens 6D of the tilt sensor 6P in such amanner that the “S-shaped curve” can be successively detected withrespect to a next target recording layer. In this way, the servocontroller 6U detects the “S-shaped curve” with respect to a targetrecording layer in the recording layer stack 67 one after another whilecounting the number of detection of the “S-shaped curve” with respect tothese recording layers.

(4) When the “S-shaped curve” with respect to the target recording layerin the recording layer stack 67 is detected, the servo controller 6Ucontrols the objective lens 64 and the deformable mirror 6Q in such amanner that the laser beam is focused on the target recording layer inthe recording layer stack 67, and controls the objective lens 64 and thedeformable mirror 6Q in such a manner that the defocus aberration outputand the spherical aberration output from the even symmetrical aberrationsensor 6S are cancelled. Also, the defocus aberration/sphericalaberration cancel controller 6I moves the condenser lens 6D of the tiltsensor 6P in the same direction as mentioned in (3). The defocusaberration/spherical aberration cancel controller 6I controls thedeformable mirror 6A in such a manner that the spherical aberrationsignal 6K detected by the aberration mode detecting circuit 6H iscanceled. Thus, the servo controller 6U detects the “S-shaped curve withrespect to the reflecting layer 68” while counting the number ofdetection of the “S-shaped curve with respect to the recording layers”one after another by implementing the aforementioned operations.

(5) When the “S-shaped curve with respect to the reflecting layer 68” isdetected, the defocus aberration/spherical aberration cancel controller6I controls the deformable mirror 6A and the condenser lens 6D in such amanner that the focusing spot regarding the “S-shaped curve with respectto the reflecting layer 68” using the defocus aberration detectionsignal 6J is controllable. Thus, tilt of the target recording layer inthe recording layer stack 67 is detected based on the X-,Y-tiltdetection signals 6N outputted from the aberration mode detectingcircuit 6H.

After the operation (5), the servo controller 6U detects the “S-shapedcurve” with respect to the target recording layer in the recording layerstack 67 based on the defocus aberration output from the evensymmetrical aberration sensor 6S, detects the “S-shaped curve” withrespect to the target recording layer in the recording layer stack 67based on the spherical aberration output from the even symmetricalaberration sensor 6S, and controls the objective lens 64 and thedeformable mirror 6Q in such a manner that the laser beam is focused onthe target recording layer in the recording layer stack 67.Simultaneously, the defocus aberration/spherical aberration cancelcontroller 6I controls the deformable mirror 6A and the condenser lens6D in such a manner that the focusing spot regarding the “S-shaped curvewith respect to the reflecting layer 68” using the defocus aberrationdetection signal 6J is controllable. The servo controller 6U controlsthe objective lens actuator 65 to detect tilt of the target recordinglayer in the recording layer stack 67 based on the X-,Y-tilt detectionsignals 6N obtained at that time.

In the above example of the operation, both the detection signals fromthe tilt sensor 6P and from the even symmetrical aberration sensor 6Sare used. Alternatively, it is possible to use the tilt sensor 6P in atime-sharing manner without using the even symmetrical aberration sensor6S and to perform an initial operation of tilt detection merely by thetilt sensor 6P.

In this embodiment, tilt of the target recording layer on which thelaser beam is focused within the recording layer stack 67 is detected bydetecting tilt aberration or coma aberration of the reflected laser beamfrom the reflecting layer 68. Alternatively, it is possible to performtilt detection by detecting tilt aberration or coma aberration of alaser beam reflected from a recording layer other than the targetrecording layer on which the laser beam is focused.

In a case that both of the tilt aberration and the coma aberration aredetected, if lens shift has not occurred, the sign of the tiltaberration and the sign of the coma aberration are different from eachother, whereas the sign of the tilt aberration and the sign of the comaaberration are identical to each other if lens shift has occurred. Inview of this, the servo controller 6U controls the objective lensactuator 65 in such a manner that the signs of the tilt aberration andof the coma aberration are identical to each other to eliminate lensshift.

A sensor utilizing the idea of a Hartmann sensor or an equivalent sensoris proposed as another example of the modal sensor. A Hartmann sensorcan detect a wavefront configuration in terms of a detection value, andcan calculate an aberration amount with respect to each of theaberration modes by expanding the detected wavefront configuration usingorthogonal Zernike circle polynomials (see Applied Optics/Vol. 38, No.16/1 June 1999 Aberration extraction in the Harmann test by use ofspatial filters by Carios Robledo-Sanchez). A similar operation can becarried out by a sensor capable of detecting a wavefront configurationsuch as a lateral share interfering sensor.

In the optical disk apparatus having the above arrangement, since tiltdetection is performed by using the laser beam reflected from thereflecting layer parallel to the recording layer, there is no need offorming a groove or the like in a recording layer to diffract the laserbeam. This arrangement enables to perform tilt detection with a highprecision even if the recording layer is flat.

In this section, described is a difference between tilt detectionaccording to the embodiment of the invention and tilt detectionaccording to the conventional art. In D1, similarly to the embodiment,tilt detection is performed by using a laser beam near a defocused beamspot. In D1, a laser beam on the recording layer is defocused byapplying an offset voltage to a focus control signal, and tilt detectionis performed by using the defocused reflected beam. On the other hand,in the embodiment, the reflecting layer 53 is formed away from therecording layer 55 by a certain distance, and the laser beam that hasbeen focused on the recording layer 55 and has been reflected from thereflecting layer 53 is detected to perform tilt detection. According tothe embodiment, since the laser beam is focused on the recording layer55, tilt detection and recording/reproducing can be carried outsimultaneously.

In D1, however, since the laser beam is not focused on the recordinglayer, it is impossible to perform recording or reproducing during tiltdetection. Generally, it takes a time in millisecond unit to defocus alaser beam on a target recording layer and then to focus the defocusedlaser beam on the target recording layer, as implemented in D1. In thisarrangement, it is difficult to perform tilt detection with respect toeach sector.

On the other hand, according to the tilt detection in the embodiment,since a laser beam is constantly focused on a target recording layer, itis possible to perform tilt detection simultaneously with recordinginformation on or reproducing information from the recording layer on areal-time basis. The embodiment enables to perform tilt detection withrespect to each sector, for instance.

Also, if a laser beam is defocused on a target recording layer prior tofocusing, as implemented in D1, the size of a beam spot is equivalentlyincreased. As a result, a crest value of a tracking error signal e.g. atracking signal of a push-pull type is lowered, which may cause unstabletracking. In view of this, tilt detection cannot be performed for a longtime according to the technique recited in D1.

On the other hand, according to the tilt detection in the embodiment,since a laser beam is constantly focused on a target recording layer,stable tracking can be carried out, and accordingly, tilt detection canbe performed for a long time.

For the foregoing reasons, the embodiment of the invention enables toperform tilt detection on a real-time basis, and to simultaneouslyperform tilt detection and recording/reproducing, which is notexecutable in D1.

Second Embodiment

Next, a second embodiment of the invention is described referring to thedrawings.

FIGS. 4A through 4C are illustrations for explaining a principle of tiltdetection in the second embodiment. Although FIGS. 4A through 4C show anexample that an optical disk has a single recording layer for sake ofeasy explanation, the same principle is applied to a case that anoptical disk has plural recording layers. FIG. 4A is an illustrationshowing an optical path of a laser beam focused on a flat recordinglayer 23 when an optical disk is tilted. In FIG. 4A, an optical axis 21of the laser beam is tilted with respect to a normal line 22 to therecording layer 23 by a certain angle 26. The angle 26 corresponds to atilt angle. As shown in FIG. 4A, the laser beam that is incident througha base member 29 by way of a surface 27 of the optical disk is reflectedfrom the recording layer 23. Specifically, the optical path 25 passesthe points A, B′, C′, D′ and, E′. The length of the optical path 25 isthe same between the case that the optical disk is not tilted and thecase that the optical disk is tilted. Accordingly, tilt of the opticaldisk cannot be detected with use of the reflected laser beam from therecording layer 23. Therefore, in this embodiment, as shown in FIG. 4B,a dispersing part 24 where the incident beam is dispersed is formed in apart of the recording layer 23, so that tilt detection is performedbased on the dispersed beams on the dispersing part 24. In FIG. 4B, thelaser beam passing the point A is focused on the point C′ via the pointB′, and the laser beam focused on the point C′ is dispersed as thedispersed beams on the dispersing part 24. The dispersed beams on thedispersing part 24 absorb a light energy of the incident laser beam of awavelength λ0, and are substantially symmetrically irradiated with alarge angle with respect to the normal line 22 to the recording layer 23with each one of the dispersed beams having the same wavelength λ0 asthe incident laser beam. Since the dispersed beams have no or lesscorrelation to the phase of an incoming beam, namely, the incident laserbeam that has passed through the points A, B′, and C′, the dispersedbeams have tilt aberration and coma aberration. Accordingly, tilt of theoptical disk can be detected based on the tilt aberration and the comaaberration of the dispersed beams. FIG. 4C shows a case that adispersing layer 28 which has a certain positional relation to therecording layer 23 and which is adapted to disperse the incident beam isformed. In FIG. 4C, the dispersing layer 28 is formed parallel to therecording layer 23. In FIG. 4C, tilt of the recording layer 23 isindirectly detected by performing tilt detection using the dispersedbeams on the dispersing layer 28. In FIG. 4C, a laser beam passing thepoint A is focused on the point C′ via the point B′. Since the point C′lies on the dispersing layer 28, the dispersed beams absorb a lightenergy of the incident laser beam of the wavelength λ0, and aresubstantially symmetrically irradiated with a large angle with respectto the normal line 22 to the recording layer 23 with each one of thedispersed beams having the same wavelength λ0 as the incident laserbeam. Similarly to the case shown in FIG. 4B, in FIG. 4C, since thedispersed beams have no or less correlation to the phase of an incomingbeam, namely, the incident laser beam that has passed through the pointsA, B′, and C′, tilt of the dispersing layer 28 can be detected by tiltaberration and coma aberration of the dispersed beams, and tilt of therecording layer 23 can be indirectly detected based on the tilt of thedispersing layer 28. An example as to how the dispersing layer 28 isformed is described in detail in a third embodiment.

FIG. 5 is an illustration for explaining an arrangement of a recordinglayer of an optical disk in the second embodiment. A dispersing part 72is formed in a part of a recording layer 71 of the optical disk. Thedispersing part 72 is formed by partly randomizing the phase of anincident laser beam.

The following is an example as to how the dispersing part 72 is formed.

1) A diffuse reflection part for diffusingly reflecting light is formedby forming asperities on a surface of a recording layer of an opticaldisk. FIG. 6A is an illustration showing an example of the diffusereflection part. Preferably, the depth of a recess or the height of aprotrusion on the diffuse reflection part, namely, a dispersing part 82is a half wavelength (λ0/2) or more, wherein λ0 is the wavelength of alaser beam. The recesses or the protrusions are formed by locally makinga surface of a stamper corresponding to the dispersing part 82 coarseand by transferring the coarse surface of the stamper onto the surfaceof the recording layer 81 corresponding to the dispersing part 82. Thelaser beam of the wavelength λ0 is turned into dispersed beams eachhaving the wavelength λ0 after diffusingly reflected from a dispersingsurface 83 of the dispersing part 82. The dispersing surface 83 isobtained by forming the recording layer 81 into asperities. Thedispersing part 82 may be constituted solely of recesses each having adepth corresponding to a half wavelength or more of the laser beamwavelength with respect to the surface of the recording layer 81, or maybe constituted solely of protrusions each having a height correspondingto a half wavelength or more of the laser beam wavelength with respectto the surface of the recording layer 81.

2) Dispersants are dispersed in a medium having transmittance to a laserbeam by a depth corresponding to at least a half wavelength or more ofthe laser beam wavelength. FIG. 5 shows an example of this technique.The medium 74 is translucent or semi-translucent to the wavelength of anincident laser beam. The dispersants 73 each have a refractive indexdifferent from that of the medium 74, and have a property that part ofthe incident laser beam is reflected from a boundary between each of thedispersants 73 and the medium 74. The dispersants 73 are substantiallycontinuously dispersed in the medium 74 by a depth corresponding to atleast the half wavelength or more.

In this arrangement, the incident laser beam is reflected from each ofthe dispersants 73 which are dispersed in the medium 74 at differentdepth positions, thereby randomizing the phase of the incident laserbeam. It is possible to use dispersants with each one having a smallersize in molecule. However, it is preferable to use dispersants eachhaving a certain molecule diameter to enhance reflecting efficiency. Aparticularly preferred example of the dispersants 73 satisfies arequirement: λ/10<D<λ/2 where λ represents the wavelength of an incidentlaser beam, and D represents an average diameter of the dispersants.

The dispersing part 72 shown in FIG. 5 may have asperities in a similarmanner as the dispersing part 82 shown in FIG. 6A. Specifically, thedispersing part 72 is formed by dispersing dispersants in a mediumhaving transmittance to a laser beam by a depth corresponding to atleast a half wavelength or more of a laser beam wavelength, and byforming the surface of the medium into asperities.

An example of the dispersants 73 includes organic materials such asvarious pigments. Alternatively, it is possible to use a material havinga high dispersability such as Intralipid® to enhance dispersability.Alternatively, it is possible to disperse an inorganic material such asvarious pigments or fullerene. As a further altered arrangement, it ispossible to irradiate a laser beam of a high output power into themedium 74 by using a laser pointer such as a YAG laser to modify amoiety of the medium 74, and to use the modified moiety of the medium 74having a different refractive index from that of the other moiety of themedium 74, as dispersants 73. FIG. 6B shows an example of thistechnique. In FIG. 6B, a laser beam of a high output power isintensively irradiated for a short time onto the recording layer 81serving as a medium, and voids 84 serving as a dispersant is formed. Inthis case, one void 84 is formed by one time irradiation of a laserbeam. Alternatively it is possible to disperse micronuclei into a mediumin advance and to form the micronuclei into a multitude of voids by onetime laser irradiation. For instance, it is possible to disperse microabsorbents having a high absorption factor of absorbing an energy of ahigh-output laser beam, as compared with a medium, into the medium asnuclei, and to irradiate a laser beam onto the absorbents. Then, solelythe absorbents are heated to a high temperature, and dispersants areformed around the absorbents due to formation of the voids or themodified moiety of the medium. In other words, the dispersant can beformed by growing micronuclei. The number of the dispersants to beformed by one time laser irradiation can be arbitrarily set by properlyregulating the amount of the absorbents which serve as the nuclei to bedispersed. Preferably, the wavelength of the high-output laser beam forforming a void or voids is different from the laser beam wavelength tobe used for recording/reproducing, and the absorbents to be used to formthe nuclei of the dispersants have a high transmittance to the laserbeam wavelength used for recording/reproducing. This arrangement enablesto simplify the process of producing an optical disk because there is noadverse influence to recording/reproducing even if the absorbents to beused to form the nuclei of the dispersant are dispersed in the entiretyof the optical disk.

It is possible to remarkably reduce the laser energy required forgrowing the nuclei into dispersants by adding a reactive chemical to theabsorbents to be used to form the nuclei. For instance, it is possibleto make a microcapsule of the absorbents to be used to form the nuclei,to encapsulate a reactive chemical which invokes a chemical reactionwith a medium into the microcapsule, to smash the microcapsule byhigh-output laser irradiation so as to modify the medium by chemicalreaction of the reactive chemical encapsulated in the microcapsule withthe medium, and to use the modified moiety of the medium as dispersants.

Alternatively, it is possible to disperse, in a medium, a photosensitivematerial which is photochemically reactive to light of a wavelengthdifferent from the laser beam wavelength to be used forrecording/reproducing, and to selectively irradiate the light during aprocess of producing an optical disk for forming a dispersing part on anintended site of the optical disk. Examples of the photosensitivematerial include photosensitive pigments which are generally used inoptical recording media such as an optical disk and a silver-halidephotograph and which have appropriate wavelength characteristics.

FIG. 7 is an illustration showing a cross section of a multilayeroptical disk in the second embodiment using the structures shown inFIGS. 5, 6A, and 6B. The optical disk in this embodiment includes anupper base member 91, a recording layer stack 92, and a lower basemember 94. The recording layer stack 92 corresponds to a portion where anumber of recording layers 95 are formed one over the other via anintermediate layer 96. A part of the respective recording layers 95 inthe recording layer stack 92 is formed into a dispersing part 93 wheredispersed beams are irradiated when a laser beam 97 is irradiated. Aphotochromic material such as diarylethene or fulgide is used as arecording material for forming the recording layers 95. Voids of about0.1 μM in average diameter or a surface formed by transferring anasperity surface with use of a stamper are used as dispersants. A UVcuring resin or ZnS—SiO₂ is used as the intermediate layers 96.

In FIG. 7, the thickness of the dispersing part 93 is set equal to thatof the corresponding recording layer 95, and the dispersing part 93 isformed at plural predetermined sites in each of the recording layers 95.Alternatively, the dispersing parts 93 in the respective recordinglayers 95 may be collectively formed in the thickness direction of therecording layers 95. Particularly, irradiating a dispersing-part-forminglaser beam over the entirety of the recording layer stack 92 tocollectively form the dispersing parts 93 in the respective recordinglayers 95 after forming the recording layer stack 92 makes it possibleto eliminate a step of forming the dispersing part 93 with respect toeach of the recording layers 95, thereby remarkably reducing the numberof steps in the process of producing an optical disk. The incidentnumerical aperture (NA) of the dispersing-part-forming laser beam is 0.3or less, preferably 0.1 or less, which is approximate to that of aparallel beam, and the configurations of the dispersing parts from theuppermost recording layer to the lowermost recording layer aresubstantially identical to each other. Examples of thedispersing-part-forming laser beam include, in addition to theaforementioned high-output laser beam, short wavelength beams such asDUV, EUV, an X-ray, a synchrotron radiation ray, and an electron beam.Use of the short wavelength beams not only enables to effectivelysuppress unduly increase of an area of the dispersing part 93 due todiffraction even with use of a small NA, but also enables to easilyinduce a change of the refractive index due to modification of therecording layers 95 and the intermediate layers 96, because therespective short wavelength beams is a high energy beam. Thisarrangement enables to select a material for the recording layers 95 andthe intermediate layers 96 from a wide range.

FIG. 8 is an illustration showing an arrangement of an optical diskapparatus in the second embodiment. The optical disk apparatus shown inFIG. 8 is an example in which the optical disk shown in FIG. 7 isapplied. An optical disk 103 for use in the optical disk apparatus isconstructed such that each of recording layers in a recording layerstack 67 has a dispersing part 101. The dispersing part 101 is formed atpredetermined sites in the number of about 10 to 50 per circumference ofthe optical disk 103, so that the optical disk apparatus candiscriminate a detection timing of the dispersing part 101 based ontiming information sent from the target recording layer.

The arrangements of a laser driving circuit 60, a laser pointer 61, acollimator lens 62, a deformable mirror 6Q, a deformable mirror drivingcircuit 6R, a deflecting beam splitter 63, an objective lens 64, anobjective lens actuator 65, the recording layer stack 67, a spindlemotor 69, a condenser lens 6D, X-,Y-tilt detection signals 6N, adeformable mirror 6Q (sic), a deformable mirror driving circuit 6R(sic), and a quarter (¼) wavelength plate 6T are the same as thecorresponding ones explained in the first embodiment, respectively.Although an even symmetrical aberration sensor 6S for correctingspherical aberration of the deformable mirror 6Q is not shown, thesensor 6S substantially equivalent to the sensor 6S in the firstembodiment is provided.

The second embodiment is different from the first embodiment in thearrangement of a tilt sensor 108. The tilt sensor 108 in the secondembodiment is different from the tilt sensor 6P in the first embodimentin that the condenser lens 6D in the tilt sensor 108 is fixed and thattilt detection is intermittently performed with respect to thedispersing parts 93. In the optical disk apparatus of this embodiment,since a laser beam is focused onto the target recording layer of theoptical disk 103, the laser beam incident onto the tilt sensor 108 doesnot have a large defocus aberration or a large spherical aberration.Therefore, in this embodiment, focusing can be performed sufficientlymerely with use of the condenser lens without cancellation of defocusaberration or spherical aberration.

In an initial state of the optical disk apparatus such as turning on ofthe power thereof, a servo controller 109 controls the objective lensactuator 65 to drive the objective lens 64 in such a manner that a laserbeam is focused onto a target recording layer in the recording layerstack 67. Since the laser beam is focused onto the target recordinglayer of the optical disk 103, the control procedure can be executedindependently of the tilt sensor 108. Specifically, the servo controller109 controls the deformable mirror 6Q based on a spherical aberrationdetection value outputted from the even symmetrical aberration sensorwhich is not shown in FIG. 8 in such a manner that spherical aberrationis correctively added for the recording layer on which the laser beam isfocused. Further, the servo controller 109 controls the objective lensactuator 65 based on a defocus aberration detection value outputted fromthe even symmetrical aberration sensor which is not shown in FIG. 8 tofocus the laser beam onto a target recording layer. Tilt of the opticaldisk is detected based on the X-, Y-tilt detection signals 6N obtainedat this time.

The dispersing parts are formed at a predetermined interval along atrack in the recording layer. For instance, in the case where trackingcontrol is executed according to a sample servo system, constituting aservo mark of the dispersants as mentioned above is equivalent to anarrangement that the dispersing parts are formed at a predeterminedinterval.

The dispersing parts may be made of a material capable of emitting lightof a wavelength different from the laser beam wavelength. For instance,a similar effect as mentioned above can be obtained by using afluorescent material such as diarylethene or fulgide in place of thedispersants, and by performing tilt detection based on fluorescenceemitted from a fluorescent portion made of the fluorescent material. Inthis case, since the laser beam incident onto the fluorescent portion,and the fluorescence are different in wavelength, it is possible toisolate the fluorescence by using an optical filter or the like for tiltdetection. This arrangement also enables to improve detectionsensitivity in tilt detection.

In the optical disk apparatus as mentioned in this embodiment, the phaseof the laser beam irradiated onto the dispersants is randomized, and thelaser beam is irradiated as dispersed beams. This arrangement enables toperform tilt detection of a high precision by detecting tilt aberrationor coma aberration of the dispersed beams.

Third Embodiment

Next, a third embodiment of the invention is described referring to thedrawings.

FIG. 9 is an illustration showing a cross section of a multilayeroptical disk in the third embodiment. The optical disk includes an upperbase member 111, a recording layer stack 112, a dispersing layer 113,and a lower base member 114. The recording layer stack 112 correspondsto a portion where a number of recording layers 115 are formed one overthe other via an intermediate layer 116. Also, the respective recordinglayers 115 in the recording layer stack 112 are formed parallel to thedispersing layer 113. These parallel layers are formed, for instance, byforming one intermediate layer 116 on the dispersing layer 113 by spincoat or sputtering, and by forming a recording layer 115 and thenanother intermediate layer 116 by spin coat or sputtering. Thus, therecording layers 115 and the dispersing layer 113 are formed parallel toeach other. A photochromic material such as diarylethene or fulgide isused for forming the recording layers 112 (sic). A UV curing resin,ZnS—SiO₂ or a like material is used for forming the intermediate layers116, for instance. The dispersing layer 113 is formed by dispersingparticles of the same material as the dispersants in the secondembodiment with each one of the particles having a maximal diametercorresponding to at least a half wavelength or less of an incident laserbeam wavelength with a predetermined density at random, for instance. Alaser beam 117 is focused on the target recording layer 115, and a laserbeam 118 is focused on the dispersing layer 113. The laser beam 117 isused for recording on or reproducing from the recording layers, and thelaser beam 118 is used for tilt detection. When the laser beam 118 isirradiated onto the dispersing layer 113, dispersed beams are irradiatedfrom the dispersing layer 113. Aberration by tilt is detected by usingthe dispersed beams.

FIG. 10 is an illustration showing an arrangement of an optical diskapparatus in the third embodiment. The optical disk apparatus shown inFIG. 10 is an example in which the optical disk shown in FIG. 9 isapplied. An optical disk 126 is the multilayer optical disk shown inFIG. 9. The optical disk 126 has a dispersing layer 125 in addition to arecording layer stack 128. The dispersing layer 125 is formed parallelto the recording layers in the recording layer stack 128. Although aneven symmetrical aberration sensor 6S for correcting sphericalaberration of a deformable mirror 6Q is not shown, the sensor 6Sequivalent to the sensor 6S in the first embodiment is provided.

A laser driving circuit (first laser driving circuit) 60 oscillates alaser beam of a wavelength λ0 by driving a laser pointer (first laserpointer) 61. A laser beam emitted from the laser pointer 61 is convertedinto parallel rays by a collimator lens (first collimator lens) 62, andthe parallel rays are incident onto the deformable mirror 6Q. In thedeformable mirror 6Q, spherical aberration correction is performed byusing the laser beam of the wavelength λ0 based on a detection valuefrom a spherical aberration sensor, which is not shown in FIG. 10.Specifically, a servo controller 130 controls the deformable mirror 6Qbased on the spherical aberration detection value outputted from theeven symmetrical aberration sensor, which is not shown in FIG. 10, insuch a manner that spherical aberration is correctively added for thetarget recording layer on which the laser beam of the wavelength λ0 isfocused. The laser beam reflected from the deformable mirror 6Q isincident onto a deflecting beam splitter 124.

A second laser driving circuit 121 oscillates a laser beam of awavelength λ1 by driving a second laser pointer 122. The wavelength λ0is different from the wavelength λ1. For instance, the wavelength λ0 is405 nm, and the wavelength λ1 is 650 nm or 780 nm. The laser beamemitted from the second laser pointer 122 is converted intosubstantially parallel rays by a second collimator lens 123, and theparallel rays are incident onto the deflecting beam splitter 124.

The laser beam of the wavelength λ0 and the laser beam of the wavelengthλ1 are emitted from a surface other than the incident surface of thedeflecting beam splitter 124. The optical axis of the laser beam of thewavelength λ0 and the optical axis of the laser beam of the wavelengthλ1 are aligned to each other. Also, the numerical aperture of the laserbeam of the wavelength λ1 is smaller than the numerical aperture of thelaser beam of the wavelength λ0.

The laser beam of the wavelength λ0 and the laser beam of the wavelengthλ1 which are emitted from the deflecting beam splitter 124 are incidentonto a deflecting beam splitter 63. The laser beam of the wavelength λ0and the laser beam of the wavelength λ1 which have been incident ontothe deflecting beam splitter 63 propagate through the deflecting beamsplitter 63, and a quarter wavelength plate 6T, and are incident onto anobjective lens 64. The objective lens 64 focuses the laser beam of thewavelength λ0 onto a target recording layer in the recording layer stack128 of the optical disk 126. Simultaneously, the objective lens 64focuses the laser beam of the wavelength λ1 onto the dispersing layer125 by position control of the second collimator lens 123 throughdriving of an actuator of the second collimator lens 123, which is notshown in FIG. 10. When the dispersing layer 125 absorbs an energy of thelaser beam of the wavelength λ1, which is irradiated by the second laserpointer 122, dispersed beams each having the wavelength λ1 go out fromthe dispersing layer 125. Part of the dispersed beams going out from thedispersing layer 125 is incident onto the objective lens 64, propagatesthrough the quarter wavelength plate 6T, and is incident onto thedeflecting beam splitter 63. Also, the laser beam of the wavelength λ0which has been focused on a target recording layer in the recordinglayer stack 128 is reflected from the target recording layer in therecording layer stack 128, is incident onto the objective lens 64,propagates through the quarter wavelength plate 6T, and is incident ontothe deflecting beam splitter 63.

After the laser beam is incident onto the deflecting beam splitter 63,the laser beam reflected from the recording layer in the recording layerstack 128, and the dispersed beams dispersed from the dispersing layer125 are reflected from the deflecting beam splitter 63 in a directiondifferent from the direction of the incoming beam, and are incident ontoan optical filter 127. The optical filter 127 has spectralcharacteristics of passing a beam of the wavelength λ0 and reflecting abeam of the wavelength λ1. In this arrangement, the reflected beam fromthe target recording layer in the optical disk 126 is not allowed topass through the optical filter 127, and is incident onto anunillustrated reproduction signal sensor. The reproduction signal sensordetects the incident beam, and reads out information recorded on thetarget recording layer. Also, the optical filter 127 passes thedispersed beams of the wavelength λ1 from the dispersing layer 125. Thedispersed beams of the wavelength λ1 which have passed through theoptical filter 127 are reflected from a reflective mirror 102, andincident onto a tilt sensor 129 indicated by the dotted line block inFIG. 10. The tilt sensor 129 shown in FIG. 10 has the same arrangementas the tilt sensor 108 shown in FIG. 8 except for the laser beamwavelength and the numerical aperture. The dispersed beams which havebeen incident onto the tilt sensor 129 are incident onto a hologram 104.Two kinds of bias X coma aberrations which are different in sign andidentical in size, and two kinds of bias Y coma aberrations which aredifferent in sign and identical in size are added to the incident laserbeam in the hologram 104. The laser beam added with the respective biasaberrations in the hologram 104 is incident onto a condenser lens 6D.The laser beam incident onto the condenser lens 6D is focused on apinhole plate 105. Four pinholes are formed in the pinhole plate 105 incorrespondence to the number of the added bias aberrations.

The laser beams that have passed through the pinholes in the pinholeplate 105 are incident onto photo-sensors of a photo-sensor array 106arrayed in correspondence to the pinholes, respectively. The laser beamsincident onto the respective photo-sensors are converted into electricalsignals, which are outputted to an aberration mode detecting circuit107. A signal from each of the photo-sensors is differentially amplifiedin the aberration mode detecting circuit 107 with respect to each of theaberration modes. The aberration mode detecting circuit 107 outputsX-,Y-tilt detection signals 6N (X-,Y-coma aberration detection signals).

Since an operation of the optical disk apparatus shown in FIG. 10 is thesame as that in FIG. 8, description thereof is omitted herein.

A laser beam is split before incident onto the optical filter 127, andthe spherical aberration sensor for detecting spherical aberration ofthe laser beam of the wavelength λ0, a defocus sensor for detectingdefocus aberration of the laser beam of the wavelength λ0, and thereproduction signal sensor, all of which are not shown in FIG. 10, areoperated based on the split laser beam.

Alternatively, it is possible to obtain a similar effect as mentionedabove by using a diffuse reflection surface or a fluorescent layer inplace of the dispersing layer 125.

Use of the optical disk apparatus as described in this embodiment isadvantageous in detecting tilt of the dispersing layer in light of thefact that dispersed beams are obtained by randomization of the phase ofthe laser beam irradiated on the dispersing layer, wherein aberrationremains merely in the dispersed beams, and that tilt aberration or comaaberration is detected by using the outgoing beam. This arrangementenables to indirectly perform tilt detection of the target recordinglayer.

Also, the dispersing layer 125 is formed at a site different from therecording layer stack 128. As compared with an arrangement that adispersing part is formed in a recording layer, this arrangementfacilitates production of an optical disk. Further, this arrangementenables to use the entire surface of the recording layer as a surfacefor recording information, and to detect aberration continuously.

Fourth Embodiment

Next, a fourth embodiment of the invention is described referring to thedrawings.

In the optical disk apparatus shown in FIG. 3, the optical disk 66 isconstructed in such a manner that tilt aberration or coma aberration ofthe reflected beam from the reflecting layer 68 is not cancelled byforming the reflecting layer 68 parallel to the recording layers in therecording layer stack 67. The optical disk apparatus and the opticaldisk in this embodiment are designed such that tilt aberration or comaaberration of a reflected beam from a recording layer is not cancelledby changing the wavefront of an incoming beam into a certain shape inplace of forming a reflecting layer. As a method for changing thewavefront of the incoming beam, there is proposed an idea of leavingspherical aberration of a certain amount or defocus aberration of acertain amount on the wavefront. In this sense, the arrangement in thefourth embodiment is substantially the same as that in FIG. 3 except forthe following two points. One is that the optical disk in the fourthembodiment does not have a reflecting layer for reflecting a defocusedlaser beam. The other is that the optical disk apparatus in the fourthembodiment does not have an even symmetrical aberration sensor 6S asshown in FIG. 3 in light of a fact that merely a laser beam reflectedfrom the recording layer is detected. The arrangement of the fourthembodiment is substantially the same as the arrangement shown in FIG. 3,but the operation in the fourth embodiment is different from that in thearrangement shown in FIG. 3 in the following point. Specifically,whereas the deformable mirror 6Q in the optical disk apparatus shown inFIG. 3 is adapted to cancel spherical aberration, a deformable mirror inthis embodiment is adapted to change the wavefront of an incoming beaminto a certain shape.

FIG. 11 is an illustration showing an arrangement of the optical diskapparatus in the fourth embodiment. FIG. 11 shows a case that atechnique of leaving spherical aberration of a certain amount in anincoming beam is applied to the optical disk apparatus. An optical disk131 corresponds to the multilayer optical disk shown in FIG. 2 exceptthat a reflecting layer corresponding to the reflecting layer 53 is notformed.

Since a laser pointer 61, a laser driving circuit 60, and a collimatorlens 62 shown in FIG. 11 have the same arrangement as the correspondingones in FIG. 3, description thereof is omitted herein. After a laserbeam is converted into parallel rays by the collimator lens 62,spherical aberration of the parallel rays are corrected by thedeformable mirror 6Q. A servo controller 6U determines a sphericalaberration correction value to be applied to the deformable mirror 6Q.Specifically, upon receiving a signal indicating a spherical aberrationamount from a tilt sensor 6P, the servo controller 6U performs apredetermined computation based on the received spherical aberrationamount to determine a spherical aberration correction value. Thespherical aberration correction value is a computed value, which allowsto leave spherical aberration of a predetermined amount. The servocontroller 6U controls the deformable mirror 6Q by way of a deformablemirror driving circuit 6R to correctively tilt the deformable mirror 6Qin accordance with the spherical aberration correction value. Executingthe feedback loop as mentioned above enables to controllably set aspherical aberration amount in the laser beams 57 which are focused onthe target recording layer 59 in the recording layer stack 52 to apredetermined amount (see FIG. 2).

The laser beam going out from the deformable mirror 6Q propagatesthrough a deflecting beam splitter 63 and a quarter (¼) wavelength plate6T, and is incident onto an objective lens 64. The servo controller 6Ucontrols an objective lens actuator 65 based on a defocus aberrationamount outputted from the tilt sensor 6P. The objective lens actuator 65drives the objective lens 64 in such a manner that the laser beam isfocused on a target recording layer in a recording layer stack 67.

The laser beam that has reached the target recording layer in therecording layer stack 67 is reflected thereon, and is returned to theobjective lens 64. At this time, tilt aberration and coma aberration inthe incoming beam and the outgoing beam, namely, the laser beam that hasbeen focused on the target recording layer in the recording layer stackand reflected thereon are not cancelled due to the existence of theaberration (in this case, spherical aberration) in the incoming beam.

The laser beam that has returned to the objective lens 64 propagatesthrough the objective lens 64 and the quarter wavelength plate 6T, isreflected from the deflecting beam splitter 63 in a direction differentfrom the direction of the incoming beam, and is incident onto the tiltsensor 6P.

The laser beam incident onto the tilt sensor 6P is incident onto adeformable mirror 6A. A deformable mirror driving circuit 6B changes themirror configuration of the deformable mirror 6A in accordance with aspherical aberration control signal 6M outputted from a defocusaberration/spherical aberration cancel controller 6I. The deformablemirror 6A cancels spherical aberration of the laser beam incident ontothe tilt sensor 6P. The spherical aberration corresponds to sphericalaberration residue of a certain amount that has been left in thedeformable mirror 6Q. The deformable mirror 6A may be omitted if a beamspot of a sufficient intensity is obtainable on a pinhole plate 6F,which will be described later, without cancellation of sphericalaberration at this stage.

The laser beam reflected from the deformable mirror 6A is incident ontoa hologram 6C. Eight kinds of bias aberrations, namely, two kinds ofbias X coma aberrations which are different in sign and identical insize, two kinds of bias Y coma aberrations which are different in signand identical in size, two kinds of bias defocus aberrations which aredifferent in sign and identical in size, and two kinds of bias sphericalaberrations which are different in sign and identical in size are addedto the incident laser beam in the hologram 6C. The aberration amounts ofthe respective bias aberrations are determined by the detectedaberration amounts, and preferably set to about half of the respectivedetected aberration amounts.

The laser beam added with the respective bias aberrations in thehologram 6C is incident onto a condenser lens 6D. The condenser lens 6Dis supported by a condenser lens actuator 6E. The condenser lensactuator 6E moves the focus position of the condenser lens 6D dependingon a defocus aberration control signal 6L outputted from the defocusaberration/spherical aberration cancel controller 6I. As mentionedabove, defocus aberration is canceled by moving the condenser lens 6D.The condenser lens actuator 6E may be omitted, and the condenser lens 6Dmay be fixed at a predetermined position if a beam spot of a sufficientintensity is obtainable on the pinhole plate 6F, which will be describedlater, without cancellation of defocus aberration at this stage.

The laser beam incident onto the condenser lens 6D is focused on thepinhole plate 6F. Eight pinholes are formed in the pinhole plate 6F incorrespondence to the number of the added bias aberrations. The radiusof each of the pinholes is for instance 1/1.22 times as large as theradius of an airy disk.

The laser beams that have passed through the pinholes in the pinholeplate 6F are incident onto photo-sensors of a photo-sensor array 6Garrayed in correspondence to the pinholes, respectively. The laser beamsincident onto the respective photo-sensors are converted into electricalsignals, which are outputted to an aberration mode detecting circuit 6H.A signal from each of the photo-sensors is differentially amplified inthe aberration mode detecting circuit 6H with respect to each of theaberration modes. Specifically, the aberration mode detecting circuit 6Houtputs X-,Y-tilt detection signals 6N (X-,Y-coma aberration detectionsignals), a defocus aberration detection signal 6J, and a sphericalaberration detection signal 6K. The X-,Y-tilt detection signals 6N(X-,Y-coma aberration detection signals) constitute an output from thetilt sensor 6P.

The defocus aberration detection signal 6J, the spherical aberrationdetection signal 6K, and the X-,Y-tilt detection signals 6N areoutputted to the defocus aberration/spherical aberration cancelcontroller 6I. The defocus aberration/spherical aberration cancelcontroller 6I generates the defocus aberration control signal 6L forcanceling the defocus aberration of the laser beam incident onto thecondenser lens 6D based on the defocus aberration signal 6J outputtedfrom the aberration mode detecting circuit 6H, and outputs the defocusaberration control signal 6L to the condenser lens actuator 6E.Likewise, the defocus aberration/spherical aberration cancel controller6I generates the spherical aberration control signal 6M for cancelingthe spherical aberration of the laser beam incident onto the deformablemirror 6A based on the spherical aberration detection signal 6Koutputted from the aberration mode detecting circuit 6H, and outputs thespherical aberration control signal 6M to the deformable mirror drivingcircuit 6B. Further, simultaneously, the defocus aberration/sphericalaberration cancel controller 6I outputs four signals i.e. the defocusaberration detection signal, the spherical aberration detection signal,and the X-,Y-tilt detection signals to the servo controller 6U. Theservo controller 6U and the defocus aberration/spherical aberrationcancel controller 6I are communicated to each other by an interactivecommunication line.

Now, an operation of the optical disk apparatus in this embodiment isdescribed. In an initial state of the optical disk apparatus such asturning on of the power of the apparatus, the servo controller 6Ucontrols the objective lens actuator 65 to move the objective lens 64 insuch a manner that a laser beam is focused on a target recording layerin the recording layer stack 67. An example of a control procedure fromthe initial state of the optical disk apparatus until a tilt detectionsignal is outputted is described as follows.

(1) The objective lens actuator 65 temporarily moves the objective lens64 to such a position that a laser beam is substantially focused on thesurface of the optical disk 66, specifically, on an uppermost recordinglayer of the recording layers in the recording layer stack 67.

(2) The servo controller 6U drives the objective lens actuator 65 toadjust the position of the objective lens 64 in such a manner that an“S-shaped curve” is detected based on the defocus aberration detectionsignal 6J with use of the laser beam reflected from the surface of theoptical disk 66, specifically, on the uppermost recording layer of therecording layers in the recording layer stack 67. Further, the defocusaberration/spherical aberration cancel controller 61 controls thecondenser lens actuator 6E of the tilt sensor 6P to adjust the positionof the condenser lens 6D. Furthermore, simultaneously, sphericalaberration is corrected with respect to the surface of the optical disk66 in such a manner that correction amounts of spherical aberration areidentical to each other between the deformable mirror 6A and thedeformable mirror 6Q.

(3) When the “S-shaped curve” is detected based on the defocusaberration detection signal 6J using the laser beam reflected from thesurface of the optical disk 66, specifically, on the uppermost recordinglayer of the recording layers in the recording layer stack 67, the servocontroller 6U moves the objective lens 64 in such a manner that thefocusing spot of the laser beam is moved in a downward direction fromthe surface of the optical disk 66. The servo controller 6U controls thedeformable mirror 6A, the deformable mirror 6Q, and the condenser lens6D of the tilt sensor 6P in such a manner that the “S-shaped curve” canbe successively detected with respect to a next target recording layerin a similar manner as in (2). In this way, the servo controller 6Udetects the “S-shaped curve” with respect to a target recording layer inthe recording layer stack 67 one after another while counting the numberof detection of the “S-shaped curve” with respect to these recordinglayers.

(4) When the “S-shaped curve” with respect to the target recording layerin the recording layer stack 67 is detected, the servo controller 6Ucontrols the objective lens 64, the deformable mirror 6Q, the condenserlens 6D, and the deformable mirror 6A based on the defocus aberrationdetection signal 6J and on the spherical aberration detection signal 6Kin the tilt sensor 6P in a similar manner as in (2).

(5) The servo controller 6U controls the deformable mirror 6A and thedeformable mirror 6Q in such a manner that a difference in sphericalaberration correction amount between the deformable mirror 6A and thedeformable mirror 6Q is set to a predetermined value. At this time, tiltof the target recording layer in the recording layer stack 67 isdetected based on the X-,Y-tilt detection signals outputted from theaberration mode detecting circuit 6H.

After the operation (5), the servo controller 6U detects the “S-shapedcurve” using the defocus aberration detection signal 6J in the tiltsensor 6P to control the objective lens 64 in such a manner that thelaser beam is focused on the target recording layer in the recordinglayer stack 67. Simultaneously, the servo controller 6U detects the“S-shaped curve” using the spherical aberration detection signal 6K inthe tilt sensor 6P to control the deformable mirror 6A and thedeformable mirror 6Q in such a manner that a difference in sphericalaberration correction amount between the deformable mirror 6A and thedeformable mirror 6Q is set to a predetermined value.

Further, as mentioned above, changing the wavefront of the incoming beammeans that the focusing spot on the target recording layer of theoptical disk 131, namely, the beam spot is not reduced to the limit ofdiffraction. This means that the beam spot may be increased with theresult that the recording density may be lowered. In view of this, thewavefront of the incoming beam is changed in a time-sharing manner at apredetermined timing to avoid an influence to the recording capacity ofuser data, for instance, on a data format, namely, on an area wheresignificant data is not recorded such as a run-in area and a run-outarea.

It should be noted that the time-sharing process is not required if thetilt detection in this embodiment is performed using a laser beam of awavelength other than the laser beam wavelength used in recording andreproducing.

Also, there is a technique of reducing the aperture in addition to thetechnique of adding defocus aberration of a certain amount and sphericalaberration of a certain amount to change the wavefront of the incomingbeam. In such an altered arrangement, the beam spot on the recordinglayer is increased by reducing the aperture. Thereby, an effectresulting from use of the asperities on the recording layer as adispersing surface, and an effect resulting from use of a recording markon the recording layer as dispersants to generate dispersed beams areadded to the effect resulting from the change of the wavefront.

In the optical disk apparatus having the above arrangement in thisembodiment, defocus aberration of a certain amount and sphericalaberration of a certain amount are added to the wavefront of the laserbeam to be irradiated onto the recording layer. This arrangement enablesto detect tilt aberration or coma aberration, which represents tilt ofthe recording layer, by using the reflected beam from the recordinglayer, even if the recording layer is flat, thereby enabling to performtilt detection of a high precision.

Fifth Embodiment

Next, a fifth embodiment of the invention is described referring to thedrawings.

The optical disk apparatuses in the first through the fourth embodimentsare designed to detect tilt of an optical disk by detecting a reflectedbeam from the optical disk. The optical disk apparatus in the fifthembodiment is designed to detect tilt of an optical disk by detectingtilt aberration or coma aberration of a laser beam which is passedthrough recording layers of an optical disk, and goes out from a surfaceof the optical disk opposite to a laser beam incident surface where thelaser beam is incident.

FIG. 12 is an illustration for explaining a principle of tilt detectionin the fifth embodiment. Although FIG. 12 shows an example that anoptical disk has a single recording layer for sake of easy explanation,the same principle is applied to a case that an optical disk has pluralrecording layers. FIG. 12 is an illustration showing an optical path ofa laser beam which is focused on a flat recording layer 44 when anoptical disk is tilted. In FIG. 12, an optical axis 40 of the laser beamis tilted with respect to a normal line 41 to the recording layer 44 bya certain angle 48. The angle 48 corresponds to a tilt angle. The laserbeam that is incident on a top surface 45 of the optical disk propagatesthrough an upper base member 42, the recording layer 44, a lower basemember 43, and goes out through a back surface 46 of the optical disk.Specifically, an optical path 47 a passes the point A1, the point B1 onthe top surface 45 of the optical disk, the point C1 on the recordinglayer 44, and the points D1, E1 on the back surface 46 of the opticaldisk. An optical path 47 b which is symmetrical to the optical path 47 awith respect to the optical axis 40 passes the point A2, the point B2 onthe top surface 45 of the optical disk, the point C on the recordinglayer 44, and the points D2, E2 on the back surface 46 of the opticaldisk. Since the optical disk is tilted, the optical path length from thepoint B1 to the point C1 is longer than the optical path length from thepoint B2 to the point C2 by the tilted amount. Since the optical disk istilted, the optical path length from the point C1 to the point D1 isshorter than the optical path length from the point C2 to the point D2by the tilted amount. In this arrangement, the optical path 47 a isshorter than the optical path 47 b.

Optical paths symmetrical with respect to the optical axis 40 have thesame correlation as mentioned above. Accordingly, asymmetricalaberration with respect to the optical axis is included in a laser beampassing through the tilted optical disk, and this asymmetricalaberration corresponds to tilt aberration or coma aberration. Unlike anarrangement that tilt is detected by using a reflected beam from arecording layer, tilt aberration or coma aberration in this case is notcancelled by the transmitted beam. As shown in FIG. 12, when the opticaldisk is tilted, tilt aberration or coma aberration is not cancelled andis included in a laser beam passing through the optical disk.

FIG. 13 is an illustration showing an arrangement of an optical diskapparatus in the fifth embodiment. FIG. 13 shows the optical diskapparatus to which a technique of detecting tilt of an optical diskusing a transmitted beam is applied. An optical disk 140 is similar tothe optical disk 66 shown in FIG. 3 except that a reflecting layercorresponding to the reflecting layer 68 is not formed, and that a laserbeam propagates through a recording layer stack 67 and goes out of thesurface of the optical disk 140 opposite to the laser beam incidentsurface where the laser beam is incident.

Since a laser pointer 61, a laser driving circuit 60, a collimator lens62, a deformable mirror 6Q, a deflecting beam splitter 63, a quarter (¼)wavelength plate 6T, an objective lens 64, and an objective lensactuator 65 shown in FIG. 13 have the same arrangements as those of thecorresponding ones in FIG. 3, description thereof is omitted herein.

A part of the laser beam focused on a target recording layer in therecording layer stack 67 propagates through the recording layer stack67, and goes out of the surface of the optical disk 140 opposite to thelaser beam incident surface.

The other part of the laser beam focused on the target recording layerin the recording layer stack 67 is reflected thereon and is returned tothe objective lens 64. The laser beam which has returned to theobjective lens 64 propagates through the objective lens 64 and thequarter wavelength plate 6T, is reflected from the deflecting beamsplitter 63 in a direction different from the direction of the incomingbeam, and is incident onto the interior of an even symmetricalaberration sensor 6S.

The laser beam which is reflected from a plane mirror 102A and isincident onto the even symmetrical aberration sensor 6S is incident ontoa hologram 6W. Four kinds of bias aberrations, namely, two kinds of biasdefocus aberrations which are different in sign and identical in size,and two kinds of bias spherical aberrations which are different in signand identical in size are added to the incident laser beam in thehologram 6W. The aberration amounts of the respective bias aberrationsare determined by the detected aberration amounts, and preferably set toabout half of the respective detected aberration amounts.

The laser beam added with the respective bias aberrations in thehologram 6W is incident onto a condenser lens 6X. The position of thecondenser lens 6X is adjusted in such a manner that a laser beam fromthe focusing spot of the objective lens 64 is focused on a pinhole plate6Y.

The laser beam incident onto the condenser lens 6X is focused on thepinhole plate 6Y. Four pinholes are formed in the pinhole plate 6Y incorrespondence to the number of the added bias aberrations in thehologram 6W. The radius of each of the pinholes is for instance 1/1.22times as large as the radius of an airy disk.

The laser beams that have passed through the pinholes in the pinholeplate 6Y are incident onto photo-sensors of a photo-sensor array 6Zarrayed in correspondence to the pinholes, respectively. The laser beamsincident onto the respective photo-sensors are converted into electricalsignals, which are outputted to an aberration mode detecting circuit610. The aberration mode detecting circuit 610 differentially amplifiesa signal from each of the photo-sensors with respect to each of theaberration modes, and outputs, to a servo controller 143, adifferentially amplified defocus aberration detection signal 14A and adifferentially amplified spherical aberration detection signal 14B. Theservo controller 143 controls a deformable mirror driving circuit 6R fordriving the deformable mirror 6Q and the objective lens actuator 65 fordriving the objective lens 64 based on the value of the defocusaberration detection signal 14A and on the value of the sphericalaberration detection signal 14B.

The laser beam which has passed through the optical disk 140 is incidentonto a transmitting-side objective lens 141. The servo controller 143controls the objective lens 64 and the transmitting-side objective lens141 in such a manner that the focusing spot of the objective lens 64 andthe focal point of the transmitting-side objective lens 141 arecoincident with each other. In this arrangement, the transmitting-sideobjective lens 141 converts a laser beam incident onto thetransmitting-side objective lens 141 into parallel rays.

The laser beam going out from the transmitting-side objective lens 141is reflected from a plane mirror 102B and is incident onto a tilt sensor144. The laser beam incident onto the tilt sensor 144 is incident onto ahologram 145. Ten kinds of bias aberrations, namely, two kinds of biasdefocus aberrations which are different in sign and identical in size,two kinds of bias X tilt aberrations which are different in sign andidentical in size, two kinds of bias Y tilt aberrations which aredifferent in sign and identical in size, two kinds of bias 0-degreeastigmatisms which are different in sign and identical in size, and twokinds of bias 45-degree astigmatism which are different in sign andidentical in size, are added to the incident laser beam in the hologram145. The aberration amounts of the respective bias aberrations aredetermined by the detected aberration amounts, and preferably set toabout half of the respective detected aberration amounts.

The laser beam added with the respective bias aberrations in thehologram 145 is incident onto a condenser lens 146. The position of thecondenser lens 146 is adjusted in such a manner that the laser beam fromthe focusing spots of the objective lens 64 and of the condenser lens146 is focused on a pinhole plate 147.

The laser beam incident onto the condenser lens 146 is focused on thepinhole plate 147. Ten pinholes are formed in the pinhole plate 147 incorrespondence to the number of the added bias aberrations in thehologram 145. The radius of each of the pinholes is for instance 1/1.22times as large as the radius of an airy disk.

The laser beams that have passed through the pinhole plate 147 areincident onto photo-sensors of a photo-sensor array 148 arrayed incorrespondence to the pinholes, respectively. The laser beams incidentonto the respective photo-sensors are converted into electrical signals,which are outputted to an aberration mode detecting circuit 149. Theaberration mode detecting circuit 149 differentially amplifies a signalfrom each of the photo-sensors with respect to each of the aberrationmodes, and outputs, to the servo controller 143, five differentdetection signals, namely, a differentially amplified defocus aberrationdetection signal, a differentially amplified X-tilt detection signal, adifferentially amplified Y-tilt detection signal, a differentiallyamplified 0-degree astigmatism detection signal, and a differentiallyamplified 45-degree astigmatism detection signal.

The optical axes of the objective lens 64 and of the transmitting-sideobjective lens 141 are coincident with each other when the 0-degreeastigmatism and the 45-degree astigmatism are minimized. Also, thefocusing spots of the objective lens 64 and of the transmitting-sideobjective lens 141 are coincident with each other when the defocusaberration is minimized. In this arrangement, the servo controller 143controls the position of the transmitting-side objective lens 141 by wayof a transmitting-side objective lens actuator 142 in such a manner thatthe 0-degree astigmatism detection signal and the 45-degree astigmatismdetection signal are minimized. Thereby, the optical axes of theobjective lens 64 and of the transmitting-side objective lens 141 aremade coincident with each other. Further, the servo controller 143controls the position of the transmitting-side objective lens 141 by wayof the transmitting-side objective lens actuator 142 in such a mannerthat the defocus aberration detection signal is minimized. Thereby, thefocusing spots of the objective lens 64 and of the transmitting-sideobjective lens 141 are made coincident with each other.

Tilt of the optical disk 140 in the X-direction and in the Y-directioncan be detected based on the X-tilt detection signal and on the Y-tiltdetection signal which are obtained when the 0-degree astigmatismdetection signal, the 45-degree astigmatism detection signal, and thedefocus aberration detection signal are minimized. The servo controller143 controls the objective lens actuator 65 and the transmitting-sideobjective lens actuator 142 based on the X-tilt detection signal and onthe Y-tilt detection signal to tilt the objective lens 64 and thetransmitting-side objective lens 141 for canceling tilt of the opticaldisk 140.

In the optical disk apparatus as mentioned above in this embodiment,part of the laser beam irradiated on the recording layer is allowed topass, and to go out of the surface of the optical disk opposite to thelaser beam incident surface. Further, tilt of the wavefront of the laserbeam which goes out of the surface of the optical disk opposite to thelaser beam incident surface is detected. This arrangement enables toperform tilt detection of the optical disk with a high precision.

Sixth Embodiment

Now, a sixth embodiment of the invention is described referring to thedrawings.

In the sixth embodiment, tilt of an optical disk is detected with a highdetection sensitivity, as compared with the second embodiment, byallowing a laser beam of a small numerical aperture (hereinafter, calledas “NA”) to disperse on a dispersant and by allowing an objective lensof a large NA to receive the dispersed beams.

FIGS. 14A and 14B are illustrations for explaining a principle of tiltdetection in the sixth embodiment. FIG. 14A shows a cross section of anoptical disk, and FIG. 14B shows an incoming beam and an outgoing beamviewed from an optical axis direction of the optical disk with respectto a plane 39 orthogonal to an optical axis 30 in FIG. 14A. AlthoughFIG. 14A shows an example that the optical disk has a single recordinglayer for sake of easy explanation, the same principle is applied to acase that an optical disk has plural recording layers.

In FIG. 14A, the optical axis 30 of the laser beam is tilted withrespect to a normal line 31 to a recording layer 32 by a certain angle36. The angle 36 corresponds to a tilt angle. A laser beam of awavelength λ0 that is incident through a base member 37 is dispersed ona dispersing part 33 of the recording layer 32, and is irradiated asdispersed beams each having a wavelength λ0. An incoming optical path 34of the incoming beam has a small numerical aperture (NA), and theincoming beam is focused on the recording layer 32. The incoming beam inthe incoming optical path 34 is dispersed on the dispersant 33 on therecording layer 32, and the dispersed beams are irradiated with a largeangle with respect to the normal line 31 to the recording layer 32. Inthis arrangement, when the optical disk apparatus receives a dispersedbeam having a larger NA than the incoming beam, the dispersed beam on anaperture area, namely, the reflected beam in an outgoing optical path 35of the outgoing beam which is not overlapped with the incoming opticalpath 34 is free from cancellation of aberration by tilt. The aperturearea corresponds to an optical path 38 free from cancellation ofaberration in FIG. 14A, namely, substantially corresponds to an outerannular zone or an orbicular zone of the outgoing optical path 35. InFIG. 14B, an optical path 3 a, which is the overlapped portion of theincoming optical path 34 having a small NA and the outgoing optical path35, is located in the center, and the outgoing optical path 35constituting part of the dispersed beams is distributed in a wide areaaround the central optical path 3 a including the central optical path 3a. The optical path 35 corresponds to the aberration-cancellation-freeaperture area or the outer annular zone of the optical path 38. In thisarrangement, tilt of the optical disk can be detected by using theoptical path 35.

FIG. 15 is an illustration showing an arrangement of an optical diskapparatus in the sixth embodiment. Since the arrangement of the sixthembodiment is substantially the same as that of the second embodiment,description on the same parts as those in the second embodiment isomitted herein. The sixth embodiment is different from the secondembodiment in the operation of the deformable mirror 6Q and in thefunction of the hologram 104.

During recording or reproducing (hereinafter, called as“recording/reproducing mode”), the deformable mirror 6Q is driven to addspherical aberration to a light component introduced through acollimator lens 62 so as to cancel spherical aberration which is addeduntil the incoming beam is focused on the recording layer. Also, thedeformable mirror 6Q is driven to irradiate a laser beam of a small NAonto a dispersing part 101 while excluding the outer annular zone of theoutgoing optical path 35 cyclically or at a predetermined timing.Hereinafter, this operation is called as “tilt detection mode”.Preferably, the NA at the recording/reproducing mode is not smaller than0.6 and not larger than 0.85. Preferably, the NA at the tilt detectionmode wherein the laser beam is incident onto the dispersing part 101excluding the outer annular zone is not smaller than 0.1 and not largerthan 0.2.

The laser beam of a small NA without incidence onto the outer annularzone by the driving of the deformable mirror 6Q, namely, the incomingbeam is passed through a deflecting beam splitter 63, a quarterwavelength plate 6T, and an objective lens 64, and is focused on atarget recording layer in the optical disk 103. The dispersing part 101is formed on the recording layer, and dispersed beams are generated onthe dispersing part 101 when the laser beam is focused on the dispersingpart 101. Tilt detection is performed by using the light componentreceived on the objective lens 64 within the dispersed beams. This lightcomponent is called as an outgoing beam. In this arrangement, the NA ofthe outgoing beam is larger than the NA of the incoming beam.Preferably, the NA of the outgoing beam is 0.6 or more.

Referring to FIG. 15, the outgoing beam is represented by theone-dotted-chain line. The outgoing beam is passed through the objectivelens 64, and the quarter wavelength plate 6T, is reflected from thedeflecting beam splitter 63, is reflected from a mirror 102, and isincident onto a tilt sensor 151.

The tilt sensor 151 is a modal sensor of the same kind as the tiltsensor 108 in the second embodiment shown in FIG. 8. The sixthembodiment is different from the second embodiment in that all the lightcomponents shown by the one-dotted-chain line in FIG. 15 are not usedfor tilt detection, and that light components corresponding to theaberration-cancellation-free area 38 shown in FIGS. 14A and 14B is used.As is obvious from FIGS. 14A and 14B, substantially all the lightcomponents in the area 38 constitute dispersed beams. Accordingly, thearea 38 is free from an influence of a non-dispersed beam, and a highS/N ratio is obtained even from a dispersed beam of a small intensity.Also, in a normal state of use, namely, in the recording/reproducingmode, defocus aberration and spherical aberration are detected.

The outgoing beam incident onto the tilt sensor 151 is incident onto ahologram 152. The hologram 152 is divided into two sections. One is anouter annular section of the hologram 152 corresponding to theaberration-cancellation-free area 38 in FIG. 14B, in which bias comaaberrations of plus sign and minus sign are applied to detect tilt ofthe optical disk in two directions, namely, X-direction and Y-direction.The other is a central section of the hologram 152 corresponding to thecentral area 3 a in FIG. 14B where the incoming optical path 34 and theoutgoing optical path 35 are overlapped, in which bias defocusaberrations of plus sign and minus sign, and bias spherical aberrationsof plus sign and minus sign are applied.

The outgoing beams added with the respective bias aberrations in thehologram 152 are passed through a condenser lens 6D, and are focused onpinholes formed in a pinhole plate 154. The beams that have passedthrough the pinhole plate 154 are incident onto photo-sensors of aphoto-sensor array 153 arrayed in correspondence to the pinholes,respectively. Detection signals in correspondence to the respectivephoto-sensors are sent from the photo-sensor array 153 to an aberrationmode detecting circuit 155. The aberration mode detecting circuit 155generates differentially amplified detection signals, wherein therespective bias aberrations of plus sign and minus sign in pair areadded, and outputs X-, Y-tilt detection signals, a defocus detectionsignal, and a spherical aberration detection signal.

In the sixth embodiment, in addition to the effect resulting fromdetecting tilt without cancellation of aberration in the outgoing beamby randomizing the phase of the incoming beam due to dispersion as shownin the second embodiment, the influence of the phase of the incomingbeam can be further reduced by using the incoming beam of a smaller NAthan the outgoing beam, thereby raising the precision in tilt detectionusing the outgoing beam. Also, the outer annular zone exclusivelyincluding the dispersed beams is used for tilt detection. Thisarrangement enables to raise the S/N ratio, thereby raising theprecision in tilt detection using the outgoing beam.

Further, in the sixth embodiment, a single laser beam is switchinglyused between the tilt detection mode and the recording/reproducing modeby using the deformable mirror 6Q in a time sequential manner. Thisarrangement enables to simplify the arrangement of the optical diskapparatus, as compared with the second embodiment.

The optical disk used in the sixth embodiment may be formed with therecording layers and the dispersing layer, as shown in FIG. 9. In suchan altered arrangement, the deformable mirror 6Q not only changes thenumerical aperture but also changes the focus position in switching overthe mode, thereby switching over the focusing beam onto the recordinglayers and the dispersing layer. In this arrangement, aberrationdetection and recording/reproducing are executable with use of a singlebeam, and the timing for irradiating a laser beam onto the dispersingpart can be arbitrarily set by the optical disk apparatus. Thisarrangement facilitates optimization regarding tradeoff between theprecision in aberration detection and a transfer speed in optical diskrecording/reproducing. For instance, in the case where the tilt angle ofthe optical disk is small, it is possible to lower the frequency ofaberration detection.

Alternatively, a dispersing part may not be formed in the optical disk.For instance, when the apparatus is in the tilt detection mode having asmall NA, the reflecting surface of an ordinary optical disk hasdispersability capable of dispersing part of an incident beam toward anouter annular zone. In this case, since a laser beam is incident with asmall numerical aperture, the diameter of the beam spot of the laserbeam on the recording layer is increased. As a result, the recordingmark itself within the beam spot is equivalently functioned asdispersing particles, thereby securing sufficiently high dispersability.

In the foregoing, described is the case that the optical disk is used.The invention, however, is widely applicable to recording media capableof recording information or reproducing information through laserirradiation, and to an apparatus and a method for controllingrecording/reproducing.

An optical disk apparatus according to an aspect of the inventioncomprises: a light source which irradiates a laser beam onto a recordinglayer of an optical disk by way of a disk base member to form a focusingspot on the recording layer, a photo detector which receives a reflectedbeam on the reflecting layer; and a tilt detecting means which detectstilt of the optical disk by using an output from the photo detector. Theoptical disk has the transparent planar disk base member, the recordinglayer formed on the disk base member, and a reflecting layer in acertain positional relation to the recording layer.

The above arrangement enables to prevent lowering of detectionsensitivity in tilt aberration due to offset of aberration of the laserbeam between the incoming beam and the outgoing beam, thereby enablingto perform tilt detection of a high precision. Also, this arrangementenables to detect tilt of the optical disk without forming a groove or apit in the recording layer of the optical disk, which effectivelysuppresses lowering of the light amount to be received on the opticaldisk due to diffraction or dispersion of the laser beam on a recordinglayer other than the target recording layer in multilayer recordingwhere recording is executed with respect to multiple recording layers.

Preferably, the recording layer may be formed closer to an incidentsurface of the optical disk where the laser beam is incident than thereflecting layer. In this arrangement, part of the laser beam that haspassed through the recording layer is reflected from the reflectinglayer, and tilt detection is performed by using the laser beam reflectedfrom the reflecting layer. This arrangement enables to perform tiltdetection while reproducing information recorded on the recording layer,or recording information onto the recording layer.

Preferably, the optical disk apparatus may further comprise anaberration canceling means which is formed on an optical path forguiding the reflected beam to the photo detector to cancel a defocusaberration and a spherical aberration of the reflected beam.

In the above arrangement, since the defocus aberration and the sphericalaberration in the reflected beam are cancelled, a clear beam spot havinga high Strehl ratio is obtained, and a detection output of a high S/Nratio is obtained, thereby enabling to improve precision in tiltdetection.

Preferably, the aberration canceling means may include a wavefrontcontrolling device which controls a wavefront of the reflected beam. Inthis arrangement, since the wavefront controlling device arbitrarilychanges the wavefront of the reflected beam, the spherical aberration inthe reflected beam can be cancelled with a simplified arrangement.

Preferably, the aberration canceling means may include a condenser lenswhich focuses the reflected beam on the photo detector, and a lensmoving means which moves the condenser lens.

In the above arrangement, since the condenser lens is moved by the lensmoving means, the defocus aberration can be cancelled by changing theposition of the condenser lens, thereby enabling to improve precision intilt detection.

An optical disk according to an aspect of the invention comprises: atransparent planar disk base member; a recording layer which is formedon the disk base member; and a reflecting layer which reflects anincident laser beam by way of the disk base member, wherein thereflecting layer is formed at a position opposing the disk base memberwith respect to the recording layer, and a gap between the recordinglayer and the reflecting layer is set larger than a wavelength of thelaser beam.

In the above arrangement, since the tilt aberration or the comaaberration is included in the laser beam reflected from the reflectinglayer without cancellation, tilt detection can be performed by using thetilt aberration or the coma aberration.

Preferably, the recording layer may be made of a photoisomerizingmaterial having a property that two-photon absorption occurs byirradiation of the laser beam. In this arrangement, merely therefractive index of the focusing spot of the laser beam on the recordinglayer made of the photoisomerizing material can be changed by utilizingthe two-photon absorption. This arrangement enables to select a targetrecording layer on which recording is intended to be performed bycontrolling the focal point of the laser beam in the depthwise directionof the multilayer optical disk.

An optical disk apparatus according to another aspect of the inventioncomprises: a light source which irradiates a laser beam onto adispersing part of an optical disk by way of a disk base member to forma focusing spot on the dispersing part, the optical disk having thetransparent planar disk base member, a recording layer formed on thedisk base member, and the dispersing part which randomizes at least apart of a phase of the laser beam incident by way of the disk basemember; a photo detector which receives a dispersed beam dispersed onthe dispersing part; and a tilt detecting means which detects tilt ofthe optical disk by using an output from the photo detector. Thisarrangement enables to prevent lowering of detection sensitivity in tiltaberration due to offset of aberration of the laser beam between theincoming beam and the outgoing beam, thereby enabling to perform tiltdetection of a high precision.

Preferably, the light source may include a first light source unit whichgenerates a first laser beam to form a first focusing spot on aninformation recording section on the recording layer by focusing thefirst laser beam onto the information recording section, and a secondlight source unit which generates a second laser beam having awavelength different from a wavelength of the first laser beam to form asecond focusing spot on the dispersing part by focusing the second laserbeam onto the dispersing part, wherein the photo detector includes afirst detecting unit which receives the first laser beam reflected fromthe first focusing spot, and a second detecting unit which receives thesecond laser beam reflected from the second focusing spot, and the tiltdetecting means detects the tilt of the optical disk by using an outputfrom the second detecting unit. The optical disk apparatus may furthercomprise a recorded information detecting means which detectsinformation recorded on the recording layer by using an output from thefirst detecting unit.

In the above arrangement, since the first laser beam for detectinginformation recorded on the recording layer, and the second laser beamfor performing tilt detection are individually irradiated, readout ofthe information recorded on the recording layer, and tilt detection canbe performed simultaneously.

An optical disk according to yet another aspect of the inventioncomprises: a transparent planar disk base member; a recording layerwhich is formed on the disk base member; and a dispersing part whichrandomizes at least a part of a phase of a laser beam incident by way ofthe disk base member.

In the above arrangement, the dispersing part randomizes the phase ofthe wavefront including the aberration generated in the incoming opticalpath of the laser beam, and tilt of the optical disk is detected byusing the reflected beam from the dispersing part. When the laser beamis dispersed, the dispersed beam has no or less correlation to the laserbeam before the dispersion, namely, the wavefront of the incoming beam.Accordingly, the light component after the dispersion, namely, theaberration in the outgoing beam is added, and the tilt aberration or thecoma aberration is not cancelled. This corresponds to a behavior oflight, wherein a point source of light is newly formed in the center ofdispersion. In this way, since the tilt aberration or the comaaberration is included in the dispersed beam dispersed on the dispersingpart without cancellation, tilt detection can be performed by using thetilt aberration or the coma aberration.

Preferably, the dispersing part may be formed on the recording layer. Inthis arrangement, since the dispersing part is formed on the recordinglayer, the laser beam can be dispersed on the dispersing part byfocusing the laser beam on the dispersing part of the recording layer.

Preferably, the dispersing part may include a servo mark formed on therecording layer. This arrangement enables to disperse the laser beam byusing the servo mark on the recording layer without forming thedispersing part for dispersing the laser beam on the recording layer.

Preferably, the dispersing part may be formed on a layer different fromthe recording layer. In this arrangement, since the dispersing part isformed on the layer different from the recording layer, as compared withan arrangement that the dispersing part is formed in the recordinglayer, production of the optical disk is easy. Also, this arrangementenables to utilize the entire surface of the recording layer as asurface for recording information, and aberration detection can becarried out continuously.

Preferably, the dispersing part includes at least one of a recess and aprotrusion formed on a surface thereof to diffusingly reflect the laserbeam on the surface of the dispersing part. This arrangement enables todiffusingly reflect the laser beam from the dispersing part having atleast one of the micro recess or the micro protrusion on the surface ofthe dispersing part.

Preferably, the depth of the recess or the height of the protrusion maybe a half wavelength of the laser beam or more. This arrangement enablesto efficiently and diffusingly reflect the laser beam.

Preferably, the dispersing part may be made of a medium havingtransmittance to the laser beam, and may be formed by dispersingdispersants capable of reflecting the laser beam in the medium from asurface of the medium to a depth thereof corresponding to a halfwavelength of the laser beam or more.

In the above arrangement, the phase of the incident laser beam israndomized when the incident laser beam is reflected from thedispersants located at various depth positions, and the tilt aberrationor the coma aberration is included in the phase-randomized laser beamwithout cancellation. This arrangement enables to perform tilt detectionby using the tilt aberration or the coma aberration.

Preferably, the dispersants may be formed by modifying the medium byselectively giving an energy larger than an energy of the laser beam tothe medium. This arrangement enables to form the dispersants bymodifying a moiety of the medium by selectively giving the energy higherthan the energy of the laser beam to the medium.

Preferably, the dispersants may be formed by dispersing absorbentsserving as nuclei of the dispersants into the medium, and by allowingthe absorbents to selectively absorb an energy of the laser beam to growthe nuclei.

The above arrangement enables to form the dispersants by dispersing theabsorbents serving as the nuclei of the dispersants into the medium, andby allowing the absorbents to selectively absorb the energy of the laserbeam to grow the nuclei.

Preferably, the dispersing part may disperse a beam having a wavelengthdifferent from the wavelength of the laser beam. In this arrangement,the beam having the wavelength different from the laser beam wavelengthis irradiated by absorbing the energy of the laser beam. As a result,the incoming beam has no or less correlation to the wavefront of theincident laser beam, and the aberration of the outgoing beam is added.Thus, the tilt aberration or the coma aberration is included in theoutgoing beam without cancellation, and tilt detection can be performedby using the tilt aberration or the coma aberration.

Preferably, the dispersed beam having the wavelength different from thelaser beam wavelength may include a fluorescence. In this arrangement, aportion where the fluorescence having the wavelength different from thelaser beam wavelength is emitted by absorbing the energy of the laserbeam focused on the recording layer is formed on the recording layer,and tilt of the recording layer where the laser beam is focused isdetected by using the fluorescence. When the fluorescence on thefluorescent portion of the recording layer is emitted by absorbing theenergy of the laser beam, the fluorescence has no or less correlation tothe incoming beam, namely, the wavefront of the incident laser beam, andthe aberration of the light after the emission of the fluorescence,namely, the aberration of the outgoing beam is added. As a result, tiltaberration or coma aberration is not cancelled. In this sense, thefluorescent portion on the recording layer where the fluorescence isemitted is one of the modifications of the dispersing part.

An optical disk apparatus according to yet another aspect of theinvention comprises: a light source which irradiates a laser beam onto arecording layer of an optical disk by way of a disk base member, theoptical disk having the transparent planar disk base member, and therecording layer formed on the disk base member; a photo detector whichreceives a reflected beam from the optical disk; an incoming beamoptical system which allows the laser beam irradiated from the lightsource to be incident onto the optical disk with a first numericalaperture; an outgoing beam optical system which allows the reflectedbeam from the optical disk to be received on the photo detector with asecond numerical aperture larger than the first numerical aperture toguide the reflected beam to the photo detector; and a tilt detectingmeans which detects tilt of the optical disk by using an output from thephoto detector.

In the above arrangement, an influence of the phase of the incoming beamcan be lessened by using, as the incoming beam, the laser beam of thenumerical aperture smaller than the numerical aperture of the outgoingbeam, thereby enabling to raise the precision in tilt detection usingthe outgoing beam. Also, since the outer annular zone where thedispersed beam is exclusively included is used for tilt detection, ahigh S/N ratio can be obtained, thereby enabling to raise the precisionin tilt detection using the outgoing beam.

Preferably, the first numerical aperture may be 0.2 or smaller, and thesecond numerical aperture may be 0.6 or larger.

The above arrangement enables to set the numerical aperture of theincoming beam smaller than the numerical aperture of the outgoing beam.Further, the precision in tilt detection using the outgoing beam can beraised by setting the numerical aperture of the incoming beam to 0.2 orsmaller and by setting the numerical aperture of the outgoing beam to0.6 or larger.

Preferably, the optical disk may include a dispersing part whichrandomizes at least a part of a phase of the reflected beam. In thisarrangement, the aberration detection and the recording/reproducing canbe executed with use of a single laser beam, and the timing ofirradiating the laser beam onto the dispersing part can be arbitrarilyset by the apparatus. This arrangement facilitates optimizationregarding tradeoff between the precision in aberration detection and atransfer speed in optical disk recording/reproducing.

Preferably, the optical disk apparatus may further comprise: a modeswitching means which switches over an operation mode of the opticaldisk apparatus in a time-sharing manner between a recording/reproducingmode of performing at least one of recording information on andreproducing information from an information recording section on therecording layer by focusing the laser beam irradiated from the lightsource onto the information recording section, and a tilt detecting modeof detecting the tilt of the optical disk by focusing the laser beamirradiated from the light source onto the dispersing part; and anumerical aperture switching means which switches over the numericalaperture of the laser beam to be incident onto the optical disk betweenthe first numerical aperture and the second numerical aperture, whereinthe numerical aperture switching means sets the numerical aperture ofthe laser beam to the first numerical aperture in response to setting ofthe operation mode of the apparatus to the tilt detection mode by themode switching means, and sets the numerical aperture of the laser beamto the second numerical aperture in response to setting of the operationmode of the apparatus to the recording/reproducing mode by the modeswitching means.

In the above arrangement, since the tilt detection mode and therecording/reproducing mode are switched over in a time-series manner bythe numerical aperture switching means (deformable mirror 6Q) with useof the single laser beam, the arrangement of the optical disk apparatuscan be simplified. The deformable mirror 6Q in the sixth embodimentcorresponds to an example of the numerical aperture switching means.

Preferably, the outgoing beam optical system may guide, to the photodetector, the reflected beam from the optical disk which lies in anouter annular zone of an outgoing optical path and has a numericalaperture larger than the first numerical aperture and not larger thanthe second numerical aperture.

In the above arrangement, tilt detection can be performed because thebeam in the outer annular zone having the numerical aperture larger thanthe first numerical aperture and not larger than the second numericalaperture is guided to the photo detector, and the aberration is includedin the beam in the outer annular zone.

An optical disk apparatus according to yet another aspect of theinvention comprises: a light source which irradiates a laser beam onto arecording layer of an optical disk by way of a disk base member to forma focusing spot on the recording layer, the optical disk having thetransparent planar disk base member, and the recording layer formed onthe disk base member; a wavefront controlling device which controls awavefront of the laser beam irradiated onto the recording layer; a photodetector which receives a reflected beam from the recording layer, thewavefront controlling device time-sharingly controlling the wavefront ofthe laser beam irradiated onto the recording layer in such a manner thata defocus aberration of a predetermined amount or a spherical aberrationof a predetermined amount is included; and a tilt detecting means whichdetects tilt of the optical disk by detecting a tilt aberration or acoma aberration included in the reflected beam by using an output fromthe photo detector.

In the above arrangement, since the defocus aberration of a certainamount and the spherical aberration of a certain amount are included inthe wavefront of the laser beam irradiated onto the recording layer, thetilt aberration or the coma aberration, which represents the tilt of therecording layer, can be detected by using the reflected beam even if therecording layer is flat. This arrangement enables to perform tiltdetection of a high precision. The deformable mirror 6Q in the fourthembodiment corresponds to an example of the wavefront controllingdevice.

Preferably, the optical disk apparatus may further comprise anaberration canceling means which is formed on an optical path forguiding the reflected beam reflected from the recording layer to thephoto detector to cancel a defocus aberration and a spherical aberrationof the reflected beam.

In the above arrangement, since the defocus aberration and the sphericalaberration in the reflected beam are cancelled, merely the aberrationnecessary for tilt detection e.g. tilt aberration or coma aberration canbe detected. The deformable mirror 6A in the fourth embodimentcorresponds to an example of the aberration canceling means.

An optical disk apparatus according to yet another aspect of theinvention is adapted to perform at least one of recording information ona recording layer of an optical disk, and reproducing information fromthe recording layer. The apparatus comprises: a light source whichirradiates a laser beam onto the recording layer by way of a disk basemember, the optical disk having the transparent planar disk base member,and the recording layer formed on the disk base member, the optical diskbeing so configured as to pass at least a part of the laser beamirradiated from the light source; a photo detector which receives thelaser beam that has passed through the optical disk; and a tiltdetecting means which detects tilt of the optical disk by using anoutput from the photo detector.

In the above arrangement, since the tilt aberration and the comaaberration of the transmitted laser beam are not cancelled, tilt of theoptical disk can be detected by detecting the tilt aberration and thecoma aberration of the transmitted laser beam.

An optical disk according to still another aspect of the inventioncomprises: a first transparent planar disk base member; a second diskbase member; and a recording layer which is formed between the firstdisk base member and the second disk base member, the recording layerbeing so configured as to pass at least a part of a laser beam that hasbeen irradiated through the first disk base member through the seconddisk base member.

In the above arrangement, since at least the part of the laser beamirradiated onto the optical disk is passed through the recording layer,tilt of the optical disk can be detected by detecting the tiltaberration and the coma aberration of the transmitted laser beam.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention hereinafterdefined, they should be construed as being included therein.

EXPLOITATION IN INDUSTRY

The optical disk (optical disk-like information recording medium)according to the invention is useful in recording or reproducing digitaldata, and is useful in an optical disk apparatus for performing at leastone of recording information on a recording layer of the optical disk,and reproducing information from the recording layer.

1-28. (canceled)
 29. An optical disk apparatus comprising: a lightsource which irradiates a laser beam onto a recording layer of anoptical disk by way of a disk base member to form a focusing spot on therecording layer, the optical disk having the transparent planar diskbase member, the recording layer formed on the disk base member, and areflecting layer in a certain positional relation to the recordinglayer; a photo detector which receives a reflected beam from thereflecting layer; and a tilt detecting means which detects tilt of theoptical disk by using an output from the photo detector.
 30. The opticaldisk apparatus according to claim 29, wherein the recording layer isformed closer to an incident surface of the optical disk where the laserbeam is incident than the reflecting layer.
 31. The optical diskapparatus according to claim 29, further comprising an aberrationcanceling means which is formed on an optical path for guiding thereflected beam to the photo detector to cancel a defocus aberration anda spherical aberration of the reflected beam.
 32. The optical diskapparatus according to claim 31, wherein the aberration canceling meansincludes a wavefront controlling device which controls a wavefront ofthe reflected beam.
 33. The optical disk apparatus according to claim31, wherein the aberration canceling means includes a condenser lenswhich focuses the reflected beam on the photo detector, and a lensmoving means which moves the condenser lens.
 34. An optical diskcomprising: a transparent planar disk base member; a recording layerwhich is formed on the disk base member; and a reflecting layer whichreflects an incident laser beam by way of the disk base member, whereinthe reflecting layer is formed at a position opposing the disk basemember with respect to the recording layer, and a gap between therecording layer and the reflecting layer is set larger than a wavelengthof the laser beam.
 35. The optical disk according to claim 34, whereinthe recording layer is made of a photoisomerizing material having aproperty that two-photon absorption occurs by irradiation of the laserbeam.
 36. An optical disk apparatus comprising: a light source whichirradiates a laser beam onto a dispersing part of an optical disk by wayof a disk base member to form a focusing spot on the dispersing part,the optical disk having the transparent planar disk base member, arecording layer formed on the disk base member, and the dispersing partwhich randomizes at least a part of a phase of the laser beam incidentby way of the disk base member; a photo detector which receives adispersed beam dispersed on the dispersing part; and a tilt detectingmeans which detects tilt of the optical disk by using an output from thephoto detector.
 37. The optical disk apparatus according to claim 36,wherein the light source includes a first light source unit whichgenerates a first laser beam to form a first focusing spot on aninformation recording section on the recording layer by focusing thefirst laser beam onto the information recording section, and a secondlight source unit which generates a second laser beam having awavelength different from a wavelength of the first laser beam to form asecond focusing spot on the dispersing part by focusing the second laserbeam onto the dispersing part, the photo detector includes a firstdetecting unit which receives the first laser beam reflected from thefirst focusing spot, and a second detecting unit which receives thesecond laser beam reflected from the second focusing spot, and the tiltdetecting means detects the tilt of the optical disk by using an outputfrom the second detecting unit, the optical disk apparatus furthercomprising a recorded information detecting means which detectsinformation recorded on the recording layer by using an output from thefirst detecting unit.
 38. An optical disk comprising: a transparentplanar disk base member; a recording layer which is formed on the diskbase member; and a dispersing part which randomizes at least a part of aphase of a laser beam incident by way of the disk base member.
 39. Theoptical disk according to claim 38, wherein the dispersing part isformed on the recording layer.
 40. The optical disk according to claim39, wherein the dispersing part includes a servo mark formed on therecording layer.
 41. The optical disk according to claim 38, wherein thedispersing part is formed on a layer different from the recording layer.42. The optical disk according to claim 38, wherein the dispersing partincludes at least one of a recess and a protrusion formed on a surfacethereof to diffusingly reflect the laser beam on the surface of thedispersing part.
 43. The optical disk according to claim 42, wherein thedepth of the recess or the height of the protrusion is a half wavelengthof the laser beam or more.
 44. An optical disk according to claim 38,wherein the dispersing part is made of a medium having transmittance tothe laser beam, and is formed by dispersing dispersants capable ofreflecting the laser beam in the medium from a surface of the medium toa depth thereof corresponding to a half wavelength of the laser beam ormore.
 45. The optical disk apparatus according to claim 44, wherein thedispersants are formed by modifying the medium by selectively giving anenergy larger than an energy of the laser beam to the medium.
 46. Theoptical disk according to claim 44, wherein the dispersants are formedby dispersing absorbents serving as nuclei of the dispersants into themedium, and by allowing the absorbents to selectively absorb an energyof the laser beam to grow the nuclei.
 47. The optical disk according toclaim 38, wherein the dispersing part disperses a beam having awavelength different from the wavelength of the laser beam.
 48. Anoptical disk apparatus comprising: a light source which irradiates alaser beam onto a recording layer of an optical disk by way of a diskbase member, the optical disk having the transparent planar disk basemember, and the recording layer formed on the disk base member; a photodetector which receives a reflected beam from the optical disk; anincoming beam optical system which allows the laser beam irradiated fromthe light source to be incident onto the optical disk with a firstnumerical aperture; an outgoing beam optical system which allows thereflected beam from the optical disk to be received on the photodetector with a second numerical aperture larger than the firstnumerical aperture to guide the reflected beam to the photo detector;and a tilt detecting means which detects tilt of the optical disk byusing an output from the photo detector.
 49. The optical disk apparatusaccording to claim 48, wherein the first numerical aperture is 0.2 orsmaller, and the second numerical aperture is 0.6 or larger.
 50. Theoptical disk apparatus according to claim 48, wherein the optical diskincludes a dispersing part which randomizes at least a part of a phaseof the reflected beam.
 51. The optical disk apparatus according to claim50, further comprising: a mode switching means which switches over anoperation mode of the optical disk apparatus in a time-sharing mannerbetween a recording/reproducing mode of performing at least one ofrecording information on and reproducing information from an informationrecording section on the recording layer by focusing the laser beamirradiated from the light source onto the information recording section,and a tilt detecting mode of detecting the tilt of the optical disk byfocusing the laser beam irradiated from the light source onto thedispersing part; and a numerical aperture switching means which switchesover the numerical aperture of the laser beam to be incident onto theoptical disk between the first numerical aperture and the secondnumerical aperture, wherein the numerical aperture switching means setsthe numerical aperture of the laser beam to the first numerical aperturein response to setting of the operation mode of the apparatus to thetilt detection mode by the mode switching means, and sets the numericalaperture of the laser beam to the second numerical aperture in responseto setting of the operation mode of the apparatus to therecording/reproducing mode by the mode switching means.
 52. The opticaldisk apparatus according to claim 48, wherein the outgoing beam opticalsystem guides, to the photo detector, the reflected beam from theoptical disk which lies in an outer annular zone of an outgoing opticalpath and which has a numerical aperture larger than the first numericalaperture and not larger than the second numerical aperture.
 53. Anoptical disk apparatus comprising: a light source which irradiates alaser beam onto a recording layer of an optical disk by way of a diskbase member to form a focusing spot on the recording layer, the opticaldisk having the transparent planar disk base member, and the recordinglayer formed on the disk base member; a wavefront controlling devicewhich controls a wavefront of the laser beam irradiated onto therecording layer; a photo detector which receives a reflected beam fromthe recording layer, the wavefront controlling device time-sharinglycontrolling the wavefront of the laser beam irradiated onto therecording layer in such a manner that a defocus aberration of apredetermined amount or a spherical aberration of a predetermined amountis included; and a tilt detecting means which detects tilt of theoptical disk by detecting a tilt aberration or a coma aberrationincluded in the reflected beam by using an output from the photodetector.
 54. The optical disk apparatus according to claim 53, furthercomprising an aberration canceling means which is formed on an opticalpath for guiding the reflected beam reflected from the recording layerto the photo detector to cancel a defocus aberration and a sphericalaberration of the reflected beam.
 55. An optical disk apparatus forperforming at least one of recording information on a recording layer ofan optical disk, and reproducing information from the recording layer,the apparatus comprising: a light source which irradiates a laser beamonto the recording layer by way of a disk base member, the optical diskhaving the transparent planar disk base member, and the recording layerformed on the disk base member, the optical disk being so configured asto pass at least a part of the laser beam irradiated from the lightsource; a photo detector which receives the laser beam that has passedthrough the optical disk; and a tilt detecting means which detects tiltof the optical disk by using an output from the photo detector.
 56. Anoptical disk comprising: a first transparent planar disk base member; asecond disk base member; and a recording layer which is formed betweenthe first disk base member and the second disk base member, therecording layer being so configured as to pass at least a part of alaser beam that has been irradiated through the first disk base memberthrough the second disk base member.